KR100790638B1 - Joined body of dissimilar materials comprising steel material and aluminum material, and joining method therefor - Google Patents

Abstract

An object of the present invention is to provide a bonded body of steel and aluminum, and a spot welding method thereof, capable of performing spot welding with high bonding strength. As the transfer material bonded body 3 in which the steel material 1 and the aluminum material 2 of a specific plate | board thickness were joined by spot welding, the area of the nugget 5 in a spot welding part is related with the plate | board thickness of the aluminum material 2, In addition, while defining the area of the portion where the thickness of the interfacial reaction layer 6 in the nugget 5 is 0.5 to 10 µm in relation to the plate thickness of the aluminum material 2, high bonding strength is obtained.

Description

JOB BOND OF DISSIMILAR MATERIALS COMPRISING STEEL MATERIAL AND ALUMINUM MATERIAL, AND JOINING METHOD THEREFOR}

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a dissimilar joined body between dissimilar metal members of an iron-based material and an aluminum-based material in a transportation field such as an automobile or a railroad car, a mechanical part, a building structure, or the like, and a joining method thereof.

Spot welding generally joins metal members of the same kind, but is, for example, an iron-based material (hereinafter simply referred to as steel) and an aluminum-based material (pure aluminum and aluminum alloy). If it is applicable to joining (different joining material) of a heterogeneous metal member called only an aluminum material, it can contribute significantly to weight reduction.

However, when joining a steel material and an aluminum material, it is very difficult to obtain a joint part (bonding strength) which has reliable high strength because brittle intermetallic compounds are easily formed at the joint part. Therefore, in the past, the joining of these dissimilar joined bodies (dissimilar metal members) is performed by bolts, rivets, and the like, but there are problems such as reliability, airtightness, and cost of the joint.

Therefore, conventionally, many studies have been made about the spot welding method of these heterojunctions. For example, a method of inserting a cover plate made of an aluminum-steel clad material or a steel-based material is proposed between an aluminum material and a steel material (see Patent Documents 1, 2, 3, and 4). Moreover, the method of plating or inserting a metal with a low melting point on the steel material side is proposed (refer patent document 5, 6, 7). Moreover, the method of clamping an insulator particle between an aluminum material and a steel material (refer patent document 8), the method of previously providing an unevenness | corrugation to a member (refer patent document 9), etc. are also proposed. Moreover, the method of forming a nugget part in the interface of a steel plate and an aluminum alloy plate (refer patent document 10) etc. is also proposed by shifting the nugget part formed to an anode (aluminum) side by a polarity effect.

Patent Document 1: Japanese Patent Application Laid-Open No. 1994-63763

Patent Document 2: Japanese Patent Publication No. 1995-178563

Patent Document 3: Japanese Patent Application Laid-Open No. 1992-55066

Patent Document 4: Japanese Patent Application Laid-Open No. 1995-328774

Patent Document 5: Japanese Patent Application Laid-Open No. 1992-251676

Patent Document 6: Japanese Patent Application Laid-Open No. 1995-24581

Patent Document 7: Japanese Patent Application Laid-Open No. 1992-14383

Patent Document 8: Japanese Patent Application Laid-Open No. 193-228643

Patent Document 9: Japanese Patent Application Laid-Open No. 1997-174249

Patent Document 1O: Japanese Patent Application Laid-Open No. 193-111776

Disclosure of the Invention

Problems to be Solved by the Invention

However, any of these methods is not a simple spot welding, but requires a separate process such as spot welding, plating or processing in a multi-layer, which requires a new facility to be assembled in a developing welding line. Increases. Moreover, any of these methods also has many operational problems, such as the welding conditions are significantly limited. In addition, many problems remain, such as reproducibility in which a high bond strength is stably obtained, and a decrease in the bond strength based on the increase in the amount of thinning of the aluminum material due to the increase in the amount of heat input required.

In addition, in spot welding, not only the strength of a nugget part but also the suppression of the crack which arises in a nugget part is important, but neither of these methods examined the generation | occurrence | production of the crack which arises in a nugget part.

The present invention has been made to solve the above problems, and the steel and aluminum which can perform spot welding with high bonding strength without newly using other materials such as the cladding material and without requiring a new separate process. It is to provide an assembly of ashes and spot welding thereof. In addition, in joining steel and aluminum materials directly by spot welding, a steel body and an aluminum joined body which can be spot welded with high bonding strength with good reproducibility and without causing new problems such as increasing the weight of aluminum materials, and their It is to provide a spot welding method. Moreover, it is providing the method of spot welding, without generating the crack of a nugget part.

Means to solve the problem

The inventors of the present invention have completed the present invention by obtaining the following findings as a result of intensive research.

In order to spot-weld the same kind of materials, such as steel materials and aluminum materials, with high joining strength, generally, it is preferable to promote formation of a nugget, and it is known that the larger the nugget area, the higher the shear strength and the cross tensile strength. In addition, since the nugget area is related to the heat input amount, the higher the amount of current and the longer the time, the larger the amount of heat input. Of course, when the nugget area becomes too large, melting reaches the surface of the material to be welded, so that spatters are generated. Thus, it is important to obtain an appropriate nugget area.

However, when joining the steel materials and the aluminum materials, the steel materials have a higher melting point, higher electrical resistance, and lower thermal conductivity than aluminum materials, so that the heat generation on the steel side becomes large, and firstly, aluminum having a low melting point is melted. Next, the surface of the steel is melted, and as a result, an Al-Fe-based soft intermetallic compound layer is formed at the interface. The intermetallic compound formed by spot bonding between steel and aluminum is divided into two layers. Al ₅ Fe ₂ -based compounds on the steel side and Al ₃ Fe or Al ₁₉ Fe ₄ Si ₂ Mn on the aluminum side It is known to form. Since the intermetallic compound is very soft, it has been conventionally found that high bonding strength is not obtained.

In addition, when melting reaches the aluminum material surface and sputtering occurs, the weight loss of the aluminum material increases, and high bonding strength is not obtained. That is, when joining the steel material and the aluminum material by spot welding, it is necessary to apply a high amount of heat input to form a certain nugget diameter in order to obtain a high bonding strength. In addition, it is also necessary to minimize the melting of the steel and to suppress the generation of spatters to a minimum amount.

For that purpose, regarding spot welding conditions, welding with a high current for a short time can suppress generation of spatters while obtaining a large nugget area. In addition, as a result of being able to suppress the melting of the steel material at the bonding interface, the interface reaction layer can be thinned and the bonding strength is increased. In the long time welding, a large nugget area is obtained, but the weight of aluminum material is large due to the generation of spatters. In addition, since the melting of the steel material increases at the joining interface and the interfacial reaction layer also becomes thick, the joining strength is low.

However, as a result of the increase in current density as a high current, the generation of spatter increases depending on the type of steel, and the interfacial reaction layer is formed thick. In other words, the higher the current, the higher the bonding strength, but there is a limit to the increase in the bonding strength. At the maximum, cross-sectional tensile testing results in interfacial failure at 1.0 kN / spot or less, and the aluminum base material cannot be broken.

In addition, unlike the conventional knowledge, the thickness of the interface reaction layer is better controlled in the optimum range, it was found how important to form the interface reaction layer of the optimum thickness range in a large area. In other words, the present inventors considered that it is also necessary to control the thickness and structure of the interface reaction layer at the bonding interface in order to bond the different materials of the steel material and the aluminum material by spot welding to obtain high bonding strength.

Therefore, the results of the investigation of the thickness of the interface reaction layer affects the bonding strength with concrete, as the behavior of the interface reaction layer, constituting the interface reaction layer, the steel material side of the Al ₅ Fe ₂ based compound layer, and the aluminum jaecheuk Al ₃ When the relationship between the thickness and the area of the layer of the Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound is controlled to the optimum range, even if the interfacial reaction layer is composed of, for example, these two-layer intermetallic compounds, the bonding strength is increased. It was found out and completed this invention.

More specifically, the size of the layer of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound on the aluminum material side as the structure of the interfacial reaction layer, particularly for the Al ₅ Fe ₂ -based compound layer on the steel side Direction thickness) and the layer of Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound controlled in this optimum range can be formed in a large area, thereby significantly improving the bonding strength. Turned out.

In addition, the present inventors found that the constituent components were different depending on the grades of the steel and aluminum materials, and the Mn and Si were found as strength enhancing elements of the interfacial reaction layer. That is, it was found that the relationship between the amount of Mn and Si of steel and the amount of Si of aluminum is closely related to the bonding strength with respect to the amount of Mn and Si, and Mn in the interfacial reaction layer. It has been found that the strength of the interfacial reaction layer is significantly increased in spot welding when the amount of Si and Si is controlled by the relationship between the amount of Mn and Si of steel and the amount of Si of aluminum.

Therefore, in the present invention, the gist of the transfer material body of steel and aluminum is a transfer material body in which a steel plate having a thickness t ₁ of 0.3 to 2.5 mm and an aluminum material having a plate thickness t ₂ of 0.5 to 2.5 mm are joined by spot welding. nugget area at a welding portion is 20 × t ₂ ^0.5 to 70 × t ^{₂ 0.5} mm ₂ in relationship with the aluminum sheet thickness t _2, the area of the thickness of the interface reaction layer is from 0.5 to 10 × 10.5㎛ part in the nugget t ₂ ^0.5 mm ² or more.

The interfacial reaction layer of the bonded body has an Al ₅ Fe ₂ -based compound layer on the steel side and an Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound on the aluminum side, respectively, and the Al ₃ Fe-based compound and Al _It is preferable that the average thickness in the nugget depth direction at the nugget center of the layer of the ₁₉ Fe ₄ Si ₂ Mn compound is 0.5 to 10 µm. The area of the portion having an average thickness in the nugget depth direction of the Al ₅ Fe ₂ compound layer in a range of 0.5 to 5 μm is preferably 10 × t ₂ ^0.5 mm ² or more, and a nugget center of the Al ₅ Fe ₂ compound layer It is preferable that the average thickness in the nugget depth direction at is 0.5 to 5 µm. In addition, at the intermediate point in the thickness direction of the interfacial reaction layer, the amount of Mn elements is 1.5 times or more in proportion to the amount of Mn elements in the steel, and the amount of Si elements is 1.1 times or more in proportion to the amount of Si elements in the aluminum material and the Si of the steel materials. It is preferable that it is 1.1 times or more by ratio with an elemental quantity. Moreover, in order to achieve the said objective, in this application, as steel materials, it is preferable to contain C: 0.05-0.5%, Mn: 0.5-3%, Si: 0.02-2.0%, As an aluminum material, Si: 0.4-2% It is preferable to include.

On the other hand, in this application, the dissimilar material joined body of steel materials and aluminum materials may previously have the plating film which consists of Zn and / or Al whose melting | fusing point is 350-950 degreeC in the film thickness of 3-15 micrometers on the joining side surface of steel materials or aluminum materials. .

Further, in order to achieve the above object, the gist of the spot welding method of a dissimilar material bonded body of steel and aluminum in the present invention is that the steel having a sheet thickness t ₁ of 0.3 to 2.5 mm and the sheet thickness t ₂ of aluminum having a thickness of 0.5 to 2.5 mm. As a spot welding method for a material joining body of ash, 2 x t ₂ ^0.5 kN to 4 x t ₂ ^0.5 kN in relation to the plate thickness t ₂ by using electrode chips each having a tip diameter of 7 mmφ or more and a tip R of 75 mmR or more. A pressing force is applied and a current of 15 x t ₂ ^0.5 to 30 x t ₂ ^0.5 kA is allowed to flow at 100 x t ₂ ^0.5 msec or less. On the other hand, in this spot welding method, an electrode chip having a tip diameter of 7 mmφ or more and a tip R of 120 mmR or more is used for both sides, and a pressing force of 2.5 x t ₂ ^0.5 kN to 4 x t ₂ ^0.5 kN in relation to the plate thickness t ₂ is used. Is applied, and a current of 18 x t ₂ ^0.5 to 30 x t ₂ ^0.5 kA is preferably flowed at 100 x t ₂ ^0.5 msec or less.

In addition, in the present application when the plated film made of Zn and / or Al having a melting point of 350 to 950 ° C. in advance has a film thickness of 3 to 15 μm on the joining surface of the steel or aluminum material, The gist of the spot welding method is to use two or more processes in which the tip diameter is 7 mm phi or more and the tip R is 75 mmR or more, and the spot welding is composed of a plurality of processes, and the welding current value and / or the welding time are different. In at least one step, a pressing force of 2 x t ₂ ^0.5 kN to 4 x t ₂ ^0.5 kN is applied in relation to the plate thickness t _{2 of the} aluminum material, and 15 x t ₂ ^0.5 to 30 x t ₂ ^0.5 kA A high current process for flowing the current so that the nugget area is in the range of 20 x t ₂ ^0.5 to 70 x t ₂ ^0.5 , and a pressing force of 2 x t ₂ ^0.5 kN to 4 x t ₂ ^0.5 kN is applied as a subsequent step, 1 × t ₂ ^0.5 to 10 × t ₂ ^0.5 kA It includes a low current process to flow 100 × t ₂ ^0.5 msec to 1000 × t ₂ ^0.5 msec.

In addition, in the present invention, in order to suppress cracking of the nugget portion, the gist of the spot welding of the dissimilar material bonded body of the steel and the aluminum material is aluminum having a sheet thickness t ₁ of 0.3 to 2.5 mm and a sheet thickness t ₂ of 0.5 to 2.5 mm. As a spot welding method of the transfer material assembly of ash, the welding current at the end of energization is made smaller than at the start of energization. On the other hand, the welding current at the end of the energization is controlled so that the average cooling rate from 600 ° C. to 200 ° C. of the nugget portion (hereinafter simply referred to as the “nugget portion”) formed on the aluminum material side is 2500 ° C./s or less. It is preferable that the minimum thickness of the nugget in the cross section of the weld joint satisfies Equation 1 below.

Effects of the Invention

According to the present invention, the interfacial reaction layer in the optimum thickness range on the aluminum material side can be formed in a large area at the time of joining the material by spot welding. Thereby, a large nugget area can also be obtained and the joint strength of a transfer material bonded body can be improved. In addition, as a result of controlling the amount of Mn and Si of the interfacial reaction layer, in the heterojunction of the steel material and the aluminum material, the bonding strength is increased without using a different material as in the prior art and without requiring a new separate process. It has the effect of high spot welding.

1 is a cross-sectional view showing one embodiment of a heterojunction of the present invention.

2 is a cross-sectional view showing another embodiment of the heterojunction of the present invention.

It is explanatory drawing which shows the aspect of the spot welding for obtaining a heterojunction.

4 is an explanatory diagram schematically illustrating FIG. 5.

It is a photograph substituted drawing which shows the cross-sectional structure of the welding interface of the cross section of the heterojunction joint part of this invention.

6 is a diagram schematically illustrating an energization pattern of a conventional method.

FIG. 7 is a schematic diagram of the temperature history of the nugget portion when energized with the pattern of FIG. 1. FIG.

8 is a diagram schematically illustrating an energization pattern of the method of the present invention.

FIG. 9 is a schematic of the temperature history of the nugget portion when energized with the pattern of FIG. 3. FIG.

10 shows an example of spot welding conditions for controlling the plate thickness ratio.

It is (a) top view and (b) side view which show the temperature measuring method of a nugget part.

12 is an example of temperature history measurement in the embodiment.

It is a cross-sectional microscope observation photograph of the joint seam of No. 18 (comparative example) in an Example.

14 is a cross-sectional microscopic photograph of the joint seam of No. 1 (example of the present invention) in the Example.

15 is a cross-sectional microscopic photograph of the joint seam of No. 25 (comparative example) in the Example.

FIG. 16 is a cross-sectional microscopic photograph of the joint seam of No. 13 (Example of the present invention) in the Example. FIG.

Explanation of the sign

1: steel sheet, 2: aluminum alloy plate, 3: heterojunction, 4: oxide film,

5: nugget, 6: interfacial reaction layer, 7, 8: electrode, 40: plating film

Best Mode for Carrying Out the Invention

(Heterozygote)

In FIG. 1, one aspect of the heterojunction prescribed | regulated by this invention is shown by sectional drawing. In FIG. 1, 3 is a dissimilar material joined body which joined the steel material (steel plate) 1 and the aluminum material (aluminum alloy plate) 2 by spot welding. 4 is an oxide film on the steel 1 surface. 5 is a nugget having a welding interface (interface reaction layer) 6 in spot welding, and has a nugget diameter indicated by an arrow in the horizontal direction in the figure. t ₁ is the thickness of the steel product, t ₂ is the thickness of the aluminum material (2), Δt denotes the minimum residual sheet thickness of the aluminum material after joining by spot welding. Fig. 1 shows the bonded state in which the generation of spatter is suppressed while securing the nugget diameter, and the bonded body of the present invention also becomes the bonded state as shown in this figure.

Another aspect of the heterojunction prescribed | regulated by this invention in FIG. 2 is shown in sectional drawing. In FIG. 2, 3 is a transfer material body which joined the steel material (steel plate) 1 and the aluminum material (aluminum alloy plate) 2 by spot welding, and 40 is the plating film previously installed in the joining side surface of the steel material 1 Except that is the same as in FIG. On the other hand, the plating film 40 is provided at least on the joining side surface of any steel or aluminum material for the purpose, but not only one (one side) of the joining side surface, but also on the surface opposite to the joining side (steel material). Or on both sides of the aluminum material.

Below, the reason for limitation of each requirement of this invention and its effect are demonstrated.

(Plate thickness of steel)

In the present invention, it is necessary that the sheet thickness t ₁ of the steel material of 0.3 to 2.5mm conjugate. If the sheet thickness t ₁ of the steel product is less than 0.3mm, it is not proper can not ensure the necessary strength and rigidity as the above-mentioned structural member or structural material. In addition, since the deformation of the steel material is large and the oxide film is easily destroyed by pressurization by spot welding, the reaction with aluminum is promoted. As a result, the intermetallic compound is easily formed. On the other hand, when it exceeds 2.5 mm, since the other joining means is employ | adopted as said structural member and structural material, there is little need to join by spot welding. Because of this, there is no need to thicken the thickness t ₁ of the steel material exceeds 2.5mm.

(Tensile strength of steel)

In the present invention, the shape and material of the steel material to be used are not particularly limited, and suitable shapes and materials such as steel sheets, steel materials, and steel pipes that are used for structural members or selected from structural member applications can be used. However, it is preferable that the tensile strength of steel materials is 400 Mpa or more. As low-strength steel, in general, there are many low alloy steels, and since an oxide film is almost iron oxide, Fe and Al spread easily, and a soft intermetallic compound is easy to form. For this reason, it is preferable that tensile strength is 400 Mpa or more, Preferably it is 500 Mpa or more.

In the present invention, the components of the steel are not limited, but in order to obtain the strength of the steel, it is preferable that the steel is high tensile steel (highten). In addition, as a component of the steel, in order to increase the hardenability and to precipitate harden, strength, which selectively contains Cr, Mo, Nb, V, Ti, etc., in addition to C, can be applied. Cr, Mo, and Nb increase hardenability to improve strength, and V and Ti improve strength by precipitation hardening. However, the addition of a large amount of these elements lowers the toughness around the welded portion, and tends to cause nugget cracking. For this reason, as a component of steel, it basically contains C: 0.05-0.5%, Mn: 0.5-3.0%, Si: 0.02-2.0% by mass%, and also Cr: 0-1%, Mo: 0 It is preferable to selectively contain 1 type (s) or 2 or more types of -0.2%, Nb: 0-0.1%, V: 0-0.1%, Ti: 0-0.1% as needed. And it is preferable that the remainder composition of these steel materials consists of Fe and an unavoidable impurity.

Although Mn and Si in steel materials are mentioned later in a figure, Mn and Si in an interface reaction layer are concentrated to a desired level, and joining strength is raised. It is believed that Mn and Si melt during welding, impeding diffusion of Fe and Al at the joining interface, thereby minimizing the formation of a soft intermetallic compound. In addition, when Mn or Si is concentrated in the oxide film on the steel surface, there is also an effect of slowing the contact between the molten aluminum produced by spot welding and the steel material to increase the barrier effect of the oxide film.

(Oxide film of steel)

It is preferable that each content of Mn and Si in the oxide film 4 of the steel surface is concentrated twice or more with respect to Mn and Si content in a base material steel material, respectively. Although the action of Mn and Si in these oxide films is not certain, the oxide film on the steel surface originally has a barrier effect of delaying contact between molten aluminum and steel. Here, it is guessed that the barrier effect of this oxide film becomes remarkably large, so that Mn and Si concentrate in an oxide film. That is, Mn and Si harden the oxide film, and it is estimated that the breakdown by pressurization of the spot welding is suppressed.

In addition, these Mn and Si in the oxide film are melted even after the oxide film itself is destroyed, thereby interfering with the diffusion of Fe and Al as described above at the bonding interface, thereby minimizing the formation of a soft intermetallic compound. It is assumed that.

If each content of Mn and Si in an oxide film is less than 2 times of each content of Mn and Si of steel materials, these effects will become small and the effect which raises joining strength will not be acquired very much. The amount of Mn and Si in the oxide film depends on the amount of Mn and Si in the steel. In this regard, in order to enhance the effect of Mn and Si thickening in the oxide film, the higher the amount of Mn and Si in the oxide film, the better, and for that purpose, the more Mn and Si are contained in the steel. However, on the other hand, a large amount (addition) of Mn and Si in the steel material lowers the toughness of the steel material around the welded portion, and nugget cracking is likely to occur. For this reason, content of 1 to 2.5% is preferable with respect to Mn of steel materials, and content of 0.5 to 1.5% is preferable with respect to Si.

On the other hand, in order to increase the Mn and Si concentration in the oxide film, the amount of Mn and Si in the steel may be kept low, and Mn and Si may be concentrated only on the steel surface. The degree of Mn and Si thickening in the oxide film can be analyzed by TEM-EDX analysis from the steel cross section.

The film thickness of the oxide film which concentrated Mn and Si may be about tens of nm-1 micrometer, and you may extremely thicken an oxide film. If it is the oxide film which concentrated Mn and Si prescribed | regulated by this invention, the said barrier effect can be acquired with such a film thickness. Moreover, if it is an oxide film of these thicknesses and components, the said barrier effect can be acquired without adversely affecting welding. By this relatively simple operation of controlling the oxide film on the steel side, the effect of spot welding with high bonding strength can be obtained without significantly changing the conditions and methods to the present on the steel side and the welding side.

(Aluminum material)

The aluminum material used by this invention does not specifically limit the kind and shape of the alloy, According to the request | requirement characteristic as each structural member, the board | plate material, the shape | molding material, the forging material, the casting material, etc. which are widely used are suitably selected. However, also with respect to the strength of the aluminum material, as in the case of the steel material, the higher one is preferable in order to suppress the deformation caused by the pressure during the spot welding. In this respect, the use of A5000 series, A6000 series, etc., which are high in strength and widely used as structural members of this kind, is optimal.

On the other hand, like Si in steel materials, Si in aluminum materials also has the effect of concentrating Si in the interface reaction layer to a desired level to increase the bonding strength. It is believed that Si also melts during welding, hinders the diffusion of Fe and Al at the joining interface and minimizes the formation of a soft intermetallic compound. Therefore, it is preferable at this point that an aluminum material contains Si in the range of 0.4 to 2%, and to select the aluminum material containing the said A6000 type | system | group etc. as such an alloy.

The sheet thickness t ₂ of the aluminum material used in the present invention is in the range of 0.5 to 2.5mm. If the plate thickness t ₂ of the aluminum material is less than 0.5 mm, the strength as a structural material is insufficient, and the nugget area is not obtained, the melt is easily reached to the surface of the aluminum material, and the spatter is easily generated, resulting in high bonding strength. I do not lose. On the other hand, if the plate thickness t ₂ of the aluminum material is more than 2.5mm, as in the case of a plate thickness of the above-described steel material, since the other bonding means as the structural member or structural material is employed, the need for joining by the spot welding little. Because of this, there is an aluminum material sheet thickness t ₂ need be increased in excess of 2.5mm.

(Area of nugget)

It is preferable that the area of the nugget 5 of the spot welded part in FIG. 1 be spot joined so as to be in the range of 20 x t ₂ ^0.5 to 70 x t ₂ ^0.5 mm ² in relation to the plate thickness t _{2 of the} aluminum material. In other words, it is necessary to select the spot welding conditions so that the nugget area is in the range of 20 x t ₂ ^0.5 to 70 x t ₂ ^0.5 mm ² .

Conventionally, when spot welding the same kind of metal material, even if the area of the nugget 5 in the spot welding part is about 20 x t ^0.5 mm ² with respect to the thickness t of the metal material, in view of strength and workability, Even economically, it is said to be the best. However, in this invention, it is set as the nugget area larger than this kind of metal material with respect to the joining of dissimilar metal materials. Sufficient joining strength is obtained by spot joining so that the area of the nugget 5 in the spot weld is in the range of 20 x t ₂ ^0.5 to 70 x t ₂ ^0.5 mm ² in relation to the plate thickness t ₂ of the aluminum material. Both economy and economy are excellent. In the case of joining dissimilar metal materials such as the present invention, the optimum nugget diameter is dependent on the plate thickness on the aluminum material side, and the influence of the plate thickness of the steel is negligibly small.

Here, when the nugget area is less than 20 x t ₂ ^0.5 mm ² and more strictly less than 30 x t ₂ ^0.5 mm ² , the joint strength is insufficient if the nugget area is too small. In addition, if the nugget area exceeds 70 x t ₂ ^0.5 mm ² , it is sufficient to obtain the joint strength, but spatter is likely to occur, and the weight of the aluminum material is large, conversely, the joint strength is lowered. Therefore, the nugget area is in the range of 20 x t ₂ ^0.5 to 70 x t ₂ ^0.5 mm ² , preferably in the range of 30 x t ₂ ^0.5 to 70 x t ₂ ^0.5 mm ² .

(Measurement of the nugget area)

The nugget area in this invention is obtained by the measurement of the area of the interface which the steel-aluminum material joins. The measuring method of joining interface area can be calculated | required by image-analyzing the aluminum material side segmented by peeling or cutting | disconnecting at the joining interface, and measuring the area of a nugget. In the case where the nugget shape is substantially circular, the junction portion may be cut and observed with an optical microscope from the cross section, and the area may be obtained by measuring the diameter at the interface of the nugget to be formed. In that case, the nugget diameter in at least two orthogonal directions is measured.

(Thickness of the interface reaction layer)

In FIG. 1, it is assumed that the interface reaction layer 6 in the nugget 5 has an area of a portion of which the thickness of the interface reaction layer 6 is 0.5 to 10.5 μm of 10 × t ₂ ^0.5 mm ² or more. Although the area regulation of the interface reaction layer of the optimum thickness is from the viewpoint of the bonding strength, unlike the conventional common sense that the thinner the better, it is controlled in the optimum range, and is also a direction to actively exist as a direction to be directed. As described above, the interfacial reaction layer in the optimum thickness range is formed with a large area in order to improve the bonding strength.

Therefore, when the area of the portion where the thickness of the interfacial reaction layer is 0.5 to 10 µm is less than 10 x t ₂ ^0.5 mm ² , more strictly less than 25 x t ₂ ^0.5 mm ² , the interfacial reaction layer in the optimum thickness range is wide. It does not become weak, but rather joining strength falls. In the part where the thickness of the interfacial reaction layer is less than 0.5 µm, the diffusion of the steel-aluminum becomes insufficient, resulting in low bonding strength. On the contrary, the thicker the thickness of the interfacial reaction layer becomes, the more fragile it is, especially in the portion exceeding 10.5㎛, the joint strength is low. For this reason, the larger the area of such an interfacial reaction layer, the lower the bonding strength as the whole joint. Therefore, in order to increase the bonding strength of the whole joint, there is a need for the area of the thickness of the interface reaction layer is from 0.5 to 10.5㎛ portion 10 × t ^{₂ 0.5} mm ₂ or more, preferably not less than 25 × t ^{₂ 0.5} mm _2.

The thickness of this interface reaction layer can also be measured by image analysis and SEM observation of the aluminum material side of the area of the interface where the steel-aluminum material is joined similarly to the said nugget area.

(Compound in Interfacial Reaction Layer)

In the present invention, it is further preferred to define an intermetallic compound (at the welding interface 6 in FIG. 1) in the transfer member after spot welding. The intermetallic compound prescribed | regulated by this invention is shown by FIG. 3, 4 which is sectional drawing of the welding interface 6 of the dissimilar body junction part end surface. FIG. 3: is a figure which simplified the cross-sectional micrograph of the welding interface 6 of the junction part cross section of FIG.

As shown in Fig. 4, at the welding interface 6, a layered Al ₅ Fe ₂ -based compound layer on the steel side, and an aluminum 3 side, granular or needle-shaped Al ₃ Fe-based compound and Al ₁₉ Fe _{Each of the 4} Si ₂ Mn-based compounds has a mixed layer. In the layer of aluminum jaecheuk Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound shown in Fig. 4 are mixed, the average of the l ₁ represents the length of the nugget depth direction of the nugget depth in the nugget center shown by a one-dot chain line direction Indicates thickness. Also it shows the S ₁ is the thickness range shown in the plane direction at the joint _{(Al 3 Fe + Al 19 Fe} 4 Si 2 Mn) area of the compound (area of the plane direction at the joint of different materials bonded body). In the Al ₅ Fe ₂ -based compound layer formed in the form of a layer on the steel side of FIG. 4, l ₂ represented by the length in the nugget depth direction represents the average thickness in the nugget depth direction at the nugget center represented by a dashed line. In addition, the S ₂ shown in a plan view in the direction of the joint represents the area (area of the plane direction at the joint of different materials bonded body) of the Al ₅ Fe ₂ based compound layer is a thickness range.

(Compound layer on the aluminum material side)

In the present invention, in order to increase the bonding strength, the average thickness l ₁ in the nugget depth direction at the nugget center of the layer of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound is preferably in the range of 0.5 to 10 µm. Do. In addition, "nugget center" as used in this invention refers to the range of nugget center ± 0.1mm.

The Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound are intermetallic compounds formed on the aluminum material side, and are formed in a granular shape or needle shape as shown in FIGS. 3 and 4. In the center part (nugget center part), the size (or length of acicular compound particle) of an individual compound particle is large, and it is a grain and a needle gradually as it goes to the edge part (left-right direction of FIGS. 3, 4) of a nugget. Decreases in size and distribution. At this end, although the density of the compound particles is small and the compound particles are scattered, they exist in an area larger than the Al ₅ Fe ₂ -based compound on the steel side.

Such Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound have an wedge effect including the effect by the above-described shape, to improve the adhesion between the aluminum material and the Al ₅ Fe ₂ -based compound layer, Increase the joint strength Such an effect is not exhibited when the layers of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound are too thin. In particular, when l ₁ is less than 0.5 µm, the wedge effect is insufficient, adhesion to the Al ₅ Fe ₂ -based compound layer is poor, and breakage between layers is likely to occur, and fracture occurs at a smooth interface. Because of this, in the present invention, and the Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based nugget center average thickness l ₁ of the nugget depth direction in the compound layer over 0.5㎛.

On the other hand, when the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound layer grow excessively and form the layer too thick, the individual compound particles become the starting point of destruction. In particular, l ₁ If exceeding 10㎛ there is a tendency to remarkably. Because of this, in the present invention, the Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based nugget center average thickness l ₁ of the nugget depth direction in the compound layer in 10㎛ below.

(Compound layer on the steel side)

In the present invention, in order to further increase the bonding strength after satisfying the intermetallic compound layer condition on the aluminum material side, the area of the portion whose average thickness l ₂ in the nugget depth direction of the Al ₃ Fe-based compound layer is in the range of 0.5 to 5 μm. S ₂ is not less than 10 × t ^{₂ 0.5} mm ₂ or more, more preferably 20 × t ^{₂ 0.5} mm ₂ is preferred. In addition, in order to further increase the bonding strength, the average thickness l ₂ in the nugget depth direction at the nugget center of the Al ₅ Fe ₂ -based compound layer is preferably in the range of 0.5 to 5 μm.

(Correlation between Compound Layer on Aluminum Material Side and Compound Layer on Steel Side)

In addition to the above-described individual definition of the compound layer on the aluminum material side and the compound layer on the steel material side, it is preferable to define the mutual relationship between the compound layer on the aluminum material side and the compound layer on the steel material side in order to further increase the bonding strength. That is, the Al ₃ Fe-based compound and Al in which the average thickness l ₂ is in the range of 0.5 to 10 μm in a portion where the average thickness l ₁ of the Al ₅ Fe ₂ -based compound layer is in the range of 0.5 to 5 μm at the weld interface of the joined body. A layer of ₁₉ Fe ₄ Si ₂ Mn compound is present, and the area S ₁ of the layer of Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ Mn compound in the average thickness range is 15 × t ₂ ^0.5 mm ² or more. In addition, it is preferable that it is 25 * t <_2> ^{0.5mm < 2>} or more.

The larger the area S ₁ of the layer of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound in the average thickness range, the higher the bonding strength. If the area S ₁ is less than 15 × t ₂ ^0.5 mm ² , in the case of the same strength, the greater the joint area of the nugget, the more likely the breaking load (bond strength) of the joint is lowered. On the other hand, when the joint area of the nugget is small, the joint is likely to break with a lower load as well. When the area S ₁ of the layer of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound in the average thickness range is 25 × t ₂ ^0.5 mm ² or more, the junction area (bonding interface) having high bonding strength is sufficiently large. This results in a greater breaking load. As a result, since the bonding interface has a higher breaking load than that of the aluminum substrate, the aluminum material side is broken without breaking the interface.

Although the above-mentioned area definition of the interfacial reaction layer of the optimum thickness is from the viewpoint of the bonding strength, the correlation between the compound layer on the aluminum material side and the compound layer on the steel side is controlled from the optimum thickness and the optimum area of the interfacial reaction layer to the optimum range. will be. For this reason, unlike the conventional common sense that the thinner the direction which this invention aims at, it is also a direction which exists actively. As described above, the present invention is based on the technical idea that the interfacial reaction layer in the optimum thickness range is formed in a large area, that is, in a wide range, in order to improve the bonding strength.

(Bond strength and fracture form)

In the case of the present invention, when the bonding strength is high, the bonding interface does not break, and the bonding portion breaks into a plug shape (the aluminum material breaks outside the range where the Al ₃ Fe-based compound layer is present). In other words, the fractured form of such a joint part has shown the height of the joint strength of this invention. On the other hand, when the bonding strength is low as in the prior art, it breaks at the bonding interface and the wedge-shaped Al ₅ Fe ₂ -based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based layer are torn, resulting in an Al ₅ Fe ₂ -based compound layer and Al ₃ Fe. It breaks between layers of a compound and an Al ₁₉ Fe ₄ Si ₂ Mn compound. In other words, the fractured form of such a joint indicates that the joint strength is low.

(Factors Affecting Bond Strength)

The contribution of each factor to the joint strength described above is summarized again.

In order to improve the joint strength, in particular, the effect of the average thickness l ₁ of the layer of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound is the highest, and the average thickness l ₂ is the optimal range (0.5 to 5 μm). The effect of the area S ₂ of the Al ₅ Fe ₂ -based compound layer is also high.

The average thickness l ₁ of the layers of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound gives adhesion to the aluminum material and the AlFe-based compound layer by the wedge effect as described above, so that the bonding force at the nugget center portion is improved. Contribute to. However, the control of the average thickness l ₁ of the layers of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound includes only the layers of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound and Al _5. There is a possibility that the interface of the Fe ₂ -based compound layer is peeled off. For this reason, there exists a possibility that a breaking load does not become so high. On the other hand, the area S ₂ of the Al ₅ Fe ₂ compound layer in the portion where the average thickness l ₂ is in the optimum range (0.5 to 5 µm) contributes to a wide range of stable adhesiveness and has an effect of increasing the breaking load of the entire spot junction. . However, stable control cannot be obtained by controlling only the area S ₂ of the Al ₅ Fe ₂ -based compound layer, and the difference in strength is large. For this reason, in addition to control of the average thickness l ₁ of the layer of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound, high bonding strength when the area S ₂ of the Al ₅ Fe ₂ -based compound layer is controlled Is definitely guaranteed.

In addition, the average thickness l ₂ in the nugget depth direction at the nugget center of the Al ₅ Fe ₂ compound layer and the area S ₁ of the layer of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound in the optimum thickness range are also in the bond strength. However, the effect of improving the bond strength is smaller than the two factors. However, even when these factors are employed alone, the effect of improving the bonding strength is small, but the average thickness l ₁ of the layers of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound and the area of the Al ₅ Fe ₂ -based compound layer In addition to the control of S ₂ , the highest bonding strength is obtained when these controls are performed.

(Measurement Method of Intermetallic Compounds)

In the present invention, the Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound layer, Al ₅ Fe ₂ -based compound layer, EDX point analysis of the cross section of the junction portion in the HAADF-STEM phase (10,000 to 20,000 times) The semiquantitative analysis by the above-mentioned method is performed, and it is detected like FIG. 4 which is the said tissue photograph. In other words, unless the joint interface is measured using the HAADF-STEM method described below, it can be said that the identification of the intermetallic compound and the precise measurement of the thickness and area of the intermetallic compound layer defined in the present invention are difficult. .

The identification of these intermetallic compounds measured the composition of each measuring point of 1-1 to 1-24 of the junction interface shown in Fig. 4 in the semiquantitative analysis, and the percentage of Al, Fe, Si, Mn, Mg (at%) We distinguish by composition when we assumed. That is, 73 to 95at% and 5 to 25at% Fe amount is the amount of Al is referred to as "Al ₃ Fe-based compound" is less than 2at% the amount of Si. In addition, when the amount of Al is 70 to 78 at%, the amount of Fe is 10 to 30 at% and the amount of Si is 2 to 15 at%, it is referred to as "Al ₁₉ Fe ₄ Si ₂ Mn-based compound". In addition, 60 to 73at%, and 25 to 35at% Fe amount is the amount of Al is less than 2at% Si amount is referred to as "Al ₅ Fe ₂ compound".

The HAADF-STEM method (High Angle Annular Dark Field-Scanning Transmission Electron Microscope) is a method of obtaining a phase signal by collecting the scattered elastic scattering electrons on the high side with a toroidal detector. The HAADF-STEM image is almost unaffected by the diffraction contrast, and the contrast is proportional to the quadratic power of the atomic number (Z), resulting in a two-dimensional map with composition information as it is. Since trace elements can be detected with good sensitivity, it is effective for analyzing the microstructure of the bonding interface.

More specifically, the average thickness of each compound layer of the interfacial reaction layer is roughly measured by SIM, which is cut at the nugget center portion of the joined body and embedded in resin so that the cross section can be observed. Subsequently, the inner portion of the nugget center and the inner boundary of the layer shown as the Al ₅ Fe ₂ -based compound, the inner and outer portions of the boundary of the presence of the layer as the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound, each compound When the depth direction length of the visible layer exceeds the upper limit, the sample is thinned by performing FIB processing on the inside and outside portions of the visible portion to the thickness that can be observed by TEM using a focused ion beam processing apparatus (FB-2000A) manufactured by Hitachi, Ltd. It serves as a sample for analysis. Using a JEOL field emission transmission electron microscope (JEM-2010F) equipped with a HAADF detector, an observation voltage was observed in a range (10,000 to 20,000 times) in a range of 100 μm at an acceleration voltage of 200 kV. EDX point analysis is performed to identify a layer of an Al ₃ Fe compound, an Al ₁₉ Fe ₄ Si ₂ Mn compound, or an Al ₅ Fe ₂ compound layer.

The thickness (length) in the depth direction of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound was determined from all of the Al ₃ Fe-based compounds and Al ₁₉ Fe ₄ Si ₂ from the HAADF-STEM image having a visual field of 100 µm. The length in the depth direction of the grain and needle identified by the Mn compound was measured and averaged. Al ₅ Fe ₂ total thickness (length) of the depth of the compound layer, has an average from the same, by measuring the thickness of 5. The above measurement was performed about all the samples for observation and analysis. In the spot welding joint using a dome chip, the thickness decreased as both compounds became an end part from the center part. Therefore, the diameter of the point (part) exceeding the upper limit of the length of the depth direction of each compound, and the diameter of the point below the lower limit were calculated | required, and it converted into the area used as the optimum thickness range of each compound.

(Composition of Interfacial Reaction Layer)

In the present invention, it is preferable to increase the bonding strength by concentrating Mn and Si to a desired level in the interface reaction layer. For this purpose, the amount of Mn elements and the amount of Si elements at the intermediate point (thickness center) in the thickness direction in the interfacial reaction layer are defined. The Mn element amount (hereinafter, simply referred to as Mn amount in the interface reaction layer) at the point in the interfacial reaction layer is preferably made 1.5 times or more relative to the Mn element amount in the steel material (in terms of the ratio of Mn element in the steel material). Do.

In addition, the amount of Si elements (hereinafter, simply referred to as the amount of Si in the interface reaction layer) at the point in the interface reaction layer is made 1.1 times or more relative to the amount of Si elements in the aluminum material (as a ratio with the amount of Si elements in the aluminum material). Moreover, it is preferable to make it 1.1 times or more with respect to the Si element amount of steel materials (in ratio with the Si element amount of steel materials).

In order to concentrate these Mn and Si in an interface reaction layer at the desired level, it is necessary to optimize Mn, Si content, spot welding conditions, etc. of steel materials and aluminum materials. In actual testing, Mn can be made up to 2.5 times and Si can be made up to 1.8 times by optimizing the spot welding conditions, and in the range of the degree of concentration obtained, the bonding strength tends to increase as the concentration of these Mn and Si is concentrated.

The degree of concentration of Mn or Si in the interfacial reaction layer can be analyzed by TEM-EDX analysis from the cross section of the weld joint or by respective secondary ionic strength analysis by SIMS, but the second order of Mn and Si by SIMS can be analyzed. Analyzing the ionic strength is recommended because of the small error. The Mn element amount / Mn element amount in the interfacial reaction layer from the ratio of Mn and Si strength at the midpoint of the interfacial reaction layer and the aluminum material, Mn in Si steel and Si strength similarly obtained by SIMS, It calculates | requires respectively as Si element amount in an interface reaction layer / Si element amount of steel materials, and Si element amount in an interface reaction layer / Si element amount of aluminum materials.

(Weight of aluminum material)

In order to secure the joint strength, it is preferable to reduce the weight of the aluminum material after joining by spot welding as small as possible. As this target, it is preferable that the minimum remaining plate thickness Δt is at least 50% of the original thickness t ₂ . More preferably, the minimum remaining plate thickness Δt is 90% or more of the original thickness t ₂ . The minimum remaining plate thickness Δt of this aluminum material can be obtained by observing with an optical microscope or SEM from the cross section, measuring the plate thickness thinning length, and taking a difference from the original plate thickness.

(Plating film)

In the present invention, a plating film made of Zn and / or Al having a melting point of 350 to 950 ° C. with a film thickness of 3 to 15 μm may be provided before welding on the joining side surface of any of the above steel materials or aluminum materials. By providing a plating film having a close melting point and an appropriate film thickness so as to be an intermediate layer between steel and aluminum, the time required to form an interfacial reaction layer, which is an intermetallic compound of steel and aluminum, is controlled to provide an appropriate interface of 0.5 to 10.5 µm. It can be set as the thickness of the reaction layer. When this point and the film thickness of a plating film are less than 3 micrometers, or the melting point of a plating film is less than 350 degreeC, a plating film melt-discharges out of the system at the beginning of joining, and formation of a soft interface reaction layer cannot be suppressed. On the contrary, when the film thickness of a plated film exceeds 15 micrometers, or melting | fusing point of a plated film exceeds 950 degreeC, a large welding heat input amount is needed for melt discharge to the system out of a plated film. For this reason, the amount of melting of aluminum materials increases, and the weight loss of aluminum increases by generation | occurrence | production of a spatter. Therefore, when forming a plating film, the film thickness is 3-15 micrometers, Preferably it is the range of 5-10 micrometers, Melting | fusing point of a plating film is 350-950 degreeC, Preferably 400-900 degreeC, More preferably, The melting point of aluminum is at least 900 ° C.

On the other hand, the film thickness of a plating film is calculated | required by cutting | disconnecting the sample after plating, embedding in resin, grinding | polishing, and SEM observation. At this time, it is preferable to measure three-point thickness with a 2000-time field of view, and average these to obtain.

As a type of plating that satisfies the above conditions, when applied to steel, the corrosion resistance of the steel can be ensured, and plating can be performed easily on both steel and aluminum, so that plating based on Zn or Al is a main component. Coating, Preferably, it is preferable to set it as the plating coating which contains 80% or more of 1 type or 2 types of Zn and Al in total.

If a plating film containing Zn or Al as a main component is provided in advance on the steel side, the corrosion resistance of the steel as a structural member can also be improved. Although steel is normally used by coating, for example, even if scratches enter the coating, Zn or Al in the plated film preferentially corrodes, thereby protecting the steel as a base material. Moreover, since the potential difference between steel materials and aluminum materials is made small, the contact corrosion of the dissimilar metal which is one of the subjects in a heterojunction can also be suppressed.

Examples of the kind of the plated film containing Zn or Al as a main component include Al, Al-Zn, Al-Si, Zn, Zn-Fe, and the like. On the other hand, in the present invention, as described above, it is preferable to form an intermetallic compound layer in which the interfacial reaction layer is not easily destroyed by concentrating Mn or Si in the interfacial reaction layer to suppress the formation of the soft intermetallic compound layer. For this purpose, it is more efficient to contain Si and Mn in the plating side previously installed. From this point of view, if Mn or Si is to be concentrated in the interfacial reaction layer, it is necessary to contain 5% by mass or more of Si or Mn in the plating containing Zn or Al as a main component. On the other hand, however, when there are too many Si and Mn in plating, a crack will tend to arise around a nugget rather. This tendency is particularly remarkable when Mn is included in plating, and in this sense, plating having Zn or Al containing Mn or Si as a main component other than the Al-Si plating film is not practical. In other words, this also shows the significance of concentrating Mn and Si in the interface reaction layer by controlling the Mn content of the steel material and the Si content of the aluminum material.

Also based on these matters, the galvanized film which contains 88 mass% or more of Zn is especially recommended in the plating film of this invention among the plating films which have said Zn and Al as a main component. Moreover, it is recommended that the galvanized film containing 8-12 mass% of Fe is further provided on the surface of steel materials on it.

When the galvanized film containing 88 mass% or more of Zn is given to a steel material surface, especially corrosion resistance of steel material is high, and a galvanized film is easy to control melting | fusing point to 350-950 degreeC. In addition, in the zinc plating containing 8-12 mass% Fe further on the zinc containing 88 mass% or more of Zn, the remaining Fe component and an aluminum material react efficiently at the time of melt-discharge of a plating film. Thereby, formation of an interface reaction layer can be controlled in a short time, and it can be set as the appropriate interface reaction layer thickness of 0.5-10.5 micrometers. Of course, the corrosion resistance is high and dissimilar metal contact corrosion can be suppressed.

In addition, you may contain other components other than said component in a plating film. However, in order that the melting | fusing point range does not deviate from the scope of this invention, and inferior corrosion resistance, it is set as the addition component and addition amount which do not produce welding defects, such as a crack in the vicinity of a nugget, further.

The plating method of steel materials and aluminum materials is not limited in the present invention, but existing plating methods such as wet and dry can be used. In particular, in the zinc plating of steel, the method of electroplating, hot-dip plating, the alloying process after hot-dip plating, etc. is recommended. In addition, in zinc plating of an aluminum material, electroplating, zinc substitution plating, etc. are illustrated.

(Spot welding)

3 illustrates one embodiment of spot welding to obtain a heterojunction. In FIG. 3, 1 is a steel plate, 2 is an aluminum alloy plate, 3 is a heterojunction, 5 is a nugget, and 7 and 8 are electrodes.

Below, each condition of spot welding for obtaining the heterojunction of this invention is demonstrated.

(Pressure)

Regarding the pressing force at the time of spot welding, in order to obtain the relatively large nugget required area and the required area of the optimum interfacial reaction layer, the average thickness of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based layer l ₁ , the average thickness in the nugget depth direction within the range of the nugget center of the Al ₅ Fe ₂ compound layer l ₂ , the area S of the layer of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound in the optimum thickness range _In order to control the area S ₂ of the ₁ , Al ₅ Fe ₂ -based compound layer and the like to be within the optimum range defined by the present invention, it is necessary to apply a relatively high pressing force.

More specifically, selected from a range of relatively high pressing force of ₂ × t 2 ^0.5 kN to 4 × t ₂ ^0.5 kN to the relationship between the thickness t ₂ of the aluminum material plate. However, even within this relatively high pressing force range, the method of generating the compound varies depending on the material and other welding conditions, and is not necessarily limited to the optimum range defined by the present invention. For this reason, it is necessary to select the optimum pressing force which falls in the optimum range prescribed | regulated by the said invention from the said relatively high range of pressurization pressures according to a raw material and other welding conditions.

On the other hand, by applying a relatively large pressing force within the above range, the electrical contact between the dissimilar materials, the electrode and the material is stabilized regardless of the shape of the electrode chip, and the molten metal in the nugget is supported by the unmelted part around the nugget, It is possible to obtain the relatively large nugget required area and the required area of the optimum interfacial reaction layer. In addition, the generation of spatters can be suppressed.

If the pressing force is less than 2 x t ₂ ^0.5 kN, the pressing force is too low to obtain such an effect. In particular, in a chip in which R is at the tip, the contact area decreases, leading to a decrease in the nugget area and an increase in the current density (= increase in the interfacial reaction layer). In addition, Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound of the average thickness of the layer l _1, Al ₅ Fe _2, the average thickness of the nugget depth in the range of nugget center ± 0.1mm-based compound layer in the direction l ₂ It is highly unlikely that a back is obtained.

On the other hand, when the pressing force increases, the nugget area tends to be small, and when the pressing force exceeds 4 x t ₂ ^0.5 kN, a current exceeding the following optimum current is required to obtain a desired nugget area, and spatter is generated. In addition, because of the growth of the interfacial reaction layer, the bonding strength is low. Moreover, since deformation of an aluminum material is large and it becomes a recessed part with a big joint trace, it is not preferable from an external appearance.

(electric current)

Regarding the current at the time of spot welding, in order to obtain the relatively large nugget required area and the required area of the optimum interfacial reaction layer, the average thickness of the layers of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound is further obtained. l ₁ , the average thickness of the nugget depth in the nugget center of the Al ₅ Fe ₂ compound layer l ₂ , the area of the layer of Al ₃ Fe compound and Al ₁₉ Fe ₄ Si ₂ Mn compound in the optimum thickness range S ₁ , Al ₅ In order to control the area S ₂ of the Fe ₂ compound layer and the like to be within the optimum range defined by the present invention, it is necessary to flow a relatively high current in a short time.

Specifically, it is necessary to flow a relatively high current of 15 x t ₂ ^0.5 to 30 x t ₂ ^0.5 kA in a short time of 100 x t ₂ ^0.5 msec or less in relation to the plate thickness t _{2 of} the aluminum material. However, even within this relatively high current or time range, the method of generating the compound varies depending on the material and other welding conditions, and is not necessarily limited to the optimum range defined by the present invention. For this reason, it is necessary to select the optimum current or time which falls in the optimum range prescribed | regulated by the said invention from the said relatively high current or time range according to a raw material and other welding conditions.

In addition, by allowing such a relatively high current to flow for a short time, the electrical contact between the dissimilar material and the electrode and the material is stabilized, and the molten metal in the nugget is supported by the unmelted portion around the nugget, so that the relatively large nugget required area and the optimum interface are maintained. The required area of the reaction layer can be obtained. In addition, the generation of spatters can be suppressed.

In the case of a low current of less than 15 x t ₂ ^0.5 kA, strictly less than 18 x t ₂ ^0.5 kA, sufficient heat input for nugget formation and growth is not obtained. For this reason, the comparatively large nugget required area and the necessary area of the optimum interfacial reaction layer cannot be obtained. In addition, Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound of the average thickness of the layer l _1, Al ₅ Fe _2, the average thickness of the nugget depth in the range of nugget center ± 0.1mm-based compound layer in the direction l ₂ It is highly unlikely that a back is obtained.

On the other hand, in the case of a high current exceeding 30 x t ₂ ^0.5 kA, extra equipment is required, which is disadvantageous in terms of work and cost. For this reason, in these respects, the current is set to 30 x t ₂ ^0.5 kA or less. Therefore, the use current is in the range of 15 x t ₂ ^0.5 to 30 x t ₂ ^0.5 kA, preferably 18 x t ₂ ^0.5 to 30 x t ₂ ^0.5 kA.

(Current time)

The energization time is a relatively short time of 100 x t ₂ msec in relation to the plate thickness t _{2 of} the aluminum material. In the case of a long time when the energization time exceeds 100 x t ₂ msec, the nugget diameter can be secured, but the bonding strength is low because spattering and growth of the interfacial reaction layer are caused. As described above, in order to control the interface reaction layer, the energization time is 100 × t ₂ msec or less, preferably 20 × t ₂ ^0.5 msec to 80 × t ₂ ^0.5 msec. However, as described above, it is necessary to select the optimum time for the compound control prescribed in the present invention to fall within the optimum range in relation to the current, depending on the material and other welding conditions.

(Electrode shape)

The shape of the electrode chip for spot welding may be any shape as long as the nugget area and the interfacial reaction layer can be obtained. The electrode chips on the steel material side and the aluminum material side may have different shapes or different sizes. However, as shown in FIG. 3, both the steel material side and the aluminum material side, the "dome-shaped" electrode tip whose tip is R is preferable. In the case of such a dome type, the tip diameter and the tip R of the electrode chip need to be 7 mmφ or more and 100 mmR or more in order to make both the current density decrease and the nugget area increase. In addition, the polarity is not specified, but when using DC spot welding, it is preferable to use the aluminum material side as the anode and the steel material side as the cathode.

On the other hand, by using both of the electrode chips having a tip diameter of 7 mm φ or more and a tip R of 120 mmR or more for both, the current density decrease and the nugget area increase can be optimally achieved. When this chip is used, a pressing force of 2.5 x t ₂ ^0.5 kN to 4 x t ₂ ^0.5 kN is applied in relation to the plate thickness t ₂ , and a current of 18 x t ₂ ^0.5 to 30 x t ₂ ^0.5 kA is applied to 100. it is preferred to flow × t ₂ ^0.5 msec or less.

The optimum bonding conditions are in the balance of each of the above-described conditions. For example, when the chip diameter, the chip R, and the pressing force are increased, and the current density decreases, the amount of current is increased accordingly to control the interface reaction layer to the optimum thickness. Should be. In addition, as long as the nugget area and the area of the interfacial reaction layer of the optimum thickness are not impaired, conditions at a lower current may be added before and after this spot welding condition to form a multi-step current pattern.

So far, each condition of spot welding for obtaining the nugget area and the interfacial reaction layer thickness of the heterojunction having the excellent welding strength specified in the present invention has been described. Next, a film having a thickness of 3 to 15 μm on the joining side surface of the steel or aluminum material is described. The spot welding method in the case where a plating film made of Zn and / or Al having a melting point of 350 to 950 ° C. in advance is provided in thickness, will be described.

As the optimum control of the interfacial reaction layer in the case where the plated coating is provided, in addition to the required nugget area and the required area of the optimum interfacial reaction layer, Mn and In order to concentrate Si, it is important to also control spot welding conditions.

On the other hand, the electrode shape and the pressing force have an effect in the same manner as described above.

(Welding current value, time)

In order to obtain the required nugget area and the required area of the optimum interfacial reaction layer, it is preferable to make the step of spot welding a high current step. By providing such a high current process, the crack of a nugget is also suppressed. However, in the case where the plating layer is attached, the optimum welding time is not specified because it depends on the type and thickness of the plating layer. Regardless of the type or thickness, it is necessary to determine the welding time based on the nugget diameter. The current value and the nugget diameter function in the same range as described above. However, only the present step can not completely melt-discharge the plating layer, so that some of the plating layer remains. Therefore, the area of the interfacial reaction layer is in the range of 0.5 to 10.5 占 퐉, so that the area becomes small, which is insufficient to obtain high strength. Therefore, in the process after that process, the low current process for the purpose of melt-discharge of plating is needed. In the low current process, a pressing force of 2 × t ₂ ^0.5 kN to 4 × t ₂ ^0.5 kN is applied, and a current of 1 × t ₂ ^0.5 to 10 × t ₂ ^0.5 kA is applied to 100 × t ₂ ^0.5 msec to 1000 × t ₂ ^0.5 msec shall flow. Although the pressing force at this time may be a value different from the previous process, the same range is required due to the above-described effect. In addition, when the current is less than 1 × t ₂ ^0.5 kA or the time is 100 × t ₂ ^0.5 msec, the effect of melt discharge of plating is insufficient. If it is more than 10 x t ₂ ^0.5 kA or ^more than 1000 x t ₂ ^0.5 msec, the nugget diameter will grow, but on the other hand, since it promotes the formation of the interfacial reaction layer, the thickness of the interfacial reaction layer is in the range of 0.5 to 10.5 占 퐉. The area which becomes is small. Therefore, it is necessary to consist of several process containing the above two process controlled by the conditions range.

Next, the spot welding method which focuses on the suppression of the crack generation of a nugget part required for the use which the crack of a nugget part becomes a problem especially in the dissimilar body joined body which joined the steel material and aluminum material by the spot welding is demonstrated.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly researched the method of spot-welding aluminum material and steel, without generating the crack of a nugget part, and found out that it is very effective to make welding current at the end of electricity supply smaller than at the time of electricity supply start, and is also suitable. The conditions were examined and the present invention was derived. Hereinafter, the welding conditions for preventing nugget cracking defined in the present invention and the reason for the definition thereof will be described in detail.

When the welding current is a constant value as in the conventional method shown in FIG. 6 or when the welding current at the end of energization is larger than at the start of energization (not shown), as shown in FIG. It is considered that it is lowered and a significant deformation occurs, so that the deformation cannot be absorbed at the nugget portion and cracks are generated.

In contrast, when the energization condition is a two-stage pattern as illustrated in FIG. 8 and the welding current at the end of energization is smaller than at the start of energization, the nugget portion is slowly cooled as shown in FIG. 9. In this way, it is thought that the thermal deformation at the time of cooling is small, and the deformation | transformation which generate | occur | produced can be absorbed in a nugget part, and a crack can be suppressed.

In this invention, it is characterized by making welding current at the time of completion of electricity supply smaller than at the time of electricity supply start, and the specific means is not specifically limited. For example, as the energization pattern, after energizing for a predetermined time with the welding current at the start of energization, a method of reducing the welding current step by step or dividing it into two or more steps (see FIG. 8) or three or more steps or a method of continuously decreasing the current value is adopted. can do. On the other hand, from the viewpoint of workability, as shown in FIG. 8, it is preferable to adopt an energization pattern composed of two steps.

The present inventors focused on the relationship between the cooling rate and cracking of the nugget portion, and then investigated whether or not the cooling rate can be suppressed in certain range if the cooling rate is specifically suppressed. As a result, it turned out that the average cooling rate from 600 degreeC to 200 degreeC of the aluminum material side nugget part formed by welding shall be 2500 degrees C / s or less. The control of the cooling rate in the above temperature range includes the solidification of the aluminum-side nugget portion, which varies depending on the alloy type of aluminum, but starts at about 600 to 500 ° C and ends at about 200 ° C. It is because it is effective to control the cooling rate of the said temperature range. Although the cooling rate in the said temperature range is so preferable that it is small, and it is more preferable to suppress to 2000 degrees C / s or less, from a viewpoint of workability etc., let it be about 500 degrees C / s as a lower limit.

The present inventors also examined the specific conditions of the welding current for making the average cooling rate from 600 degreeC to 200 degreeC of a nugget part to 2500 degrees C / s or less. As a result, as shown in FIG. 8, when the energization pattern consisting of at least two stages of energization during resistance spot welding is employed, the welding current of the second stage is controlled to 70 to 10% of the welding current of the first stage. Found valid.

This is because if the welding current of the second stage exceeds 70% of the welding current of the first stage, the effect of the present invention is not exhibited and it is almost the same as that of energizing in a one-stage pattern as in the conventional method. More preferably, the welding current of the second stage is 50% or less of the welding current of the first stage. On the other hand, if the welding current of the second stage is less than 10% of the welding current of the first stage, it is not preferable because the nugget portion is quenched when the energization condition is changed from the first stage to the second stage and cracks are likely to occur. . More preferably, the welding current in the second stage is made 20% or more of the welding current in the first stage.

Although it does not specifically limit about another specific welding current, an energization time, etc., In order to weld favorably, it is good to employ the following energization conditions. That is, when the tip diameter of the electrode chip is 6 mmφ or more and the tip R is less than 75 mmR, the aluminum material is melted and metallurgical bonding can be performed by setting the welding current of the first stage to 8 kA or more (more preferably, 10 kA or more). On the other hand, if the welding current is too high, aluminum is excessively melted to cause spatter, so that the welding current in the first step is suppressed to 18.0 x t _Al ^0.5 kA or less (more preferably 15.0 x t _Al ^0.5 kA or less). It is preferable. On the other hand, when the tip diameter of the electrode chip is 6 mmφ or more and the tip R is 75 mmR or more and less than 120 mmR, the welding current of the first step is set to 15.0 x t _Al ^0.5 to 30.0 x t _Al ^0.5 kA, and the tip diameter of the electrode chip is 6 mmφ. When the tip R is 120 mmR or more, the welding current in the first step is preferably set to 18.0 x t _Al ^0.5 to 30.0 x t _Al ^0.5 kA. (t _Al : Plate thickness of aluminum base metal (unit mm))

In the energization time of the first stage, when the tip diameter of the electrode chip is 6 mmφ or more and the tip R is less than 75 mmR, it is preferable to secure 30 msec (millisecond) or more, more preferably 40 msec or more so as to sufficiently bond, but the energization time is excessive. Even if it is long, it is unpreferable because the spatter of aluminum tends to occur, and the thickness of a nugget part decreases, and cracks tend to occur easily. It is preferable to suppress the energization time of a 1st step to 600 msec or less. On the other hand, when the tip diameter of the electrode chip is 6 mmφ or more and the tip R is 75 mmR or more, the contact area between the electrode chip and the plate increases and the spatter is more likely to occur, so that the energization time is 100 x t _Al. msec or less, Preferably it is 20 * t _Al ^0.5-80 * t _Al ^0.5 msec.

The pressing force in the first step is also dependent on the welding current when the tip diameter of the electrode tip is 6 mmφ or more and the tip R is less than 75 mmR. However, it is preferable to press 1.0 kN or more in order to ensure the bonding. However, if too much pressure because it is easy to occur ^{spatter, 1.4 × I 2 × 10 -8} kN [I: welding current (A)] can be in the following range. On the other hand, when the tip diameter of the electrode chip is 6 mmφ or more and the tip R is 75 mmR or more and less than 120 mmR, the contact area between the electrode chip and the plate increases and the current density decreases, so the pressing force is 2.0 × t _Al ^0.5. It is necessary to apply a relatively high pressing force of from 4.0 x t _Al ^0.5 kN. When the tip diameter of the electrode chip is 6 mmφ or more and the tip R is 120 mmR or more, it is necessary to apply a pressing force of 2.5 x t _Al ^0.5 to 4.0 x t _Al ^0.5 kN.

Also regarding the energization conditions of the second stage, there is no particular limitation except for controlling to 70 to 10% of the welding current of the first stage as described above. However, if the energization time is remarkably short, the cooling rate cannot be sufficiently suppressed and the quenching is performed. It is preferable to secure the energization time of the second stage to 50 msec or more since the state becomes close to. On the other hand, even if the energization time of the second stage is too long, spatter is likely to occur, and therefore it is preferable to suppress it to 600 msec or less.

Although the pressing force in a 2nd step is not specifically limited, In order to ensure joining, it is preferable to pressurize 1 kN or more. However, if it is excessively pressurized, the same problem as in the first step will occur, so it is preferable to set it to 6 kN or less.

The present inventors further examined the energization conditions at the time of welding in detail for each material. As for the aluminum material, there was almost no difference in the bonding state between the alloy types under the same energization conditions. However, with respect to steel materials, when the steel sheet is energized under the same conditions, the bonding state may be different depending on the strength level of the steel sheet, and when a steel sheet having a high strength level is used, the thickness of the nugget portion formed on the aluminum material side becomes thinner and the nuggets are made. There was a tendency to crack easily in the part. This is considered to be because a large amount of alloying elements are added to the high-strength steel sheet, and heat is rapidly generated due to the presence of the alloying elements, and aluminum is easily melted and scattered. Therefore, it has been found that it is very effective to adjust the energization conditions in accordance with the strength level of the steel sheet, and that it is better to relatively suppress the amount of energization as the strength level of the steel sheet increases. The detailed conditions are as shown below.

(I) Strength level of steel: less than 390 MPa

(I-1) When the tip diameter of the electrode chip is 6 mmφ or more and the tip R is less than 75 mmR

<Step 1 condition>

Current amount (I): 18.0 x t _Al ^0.5 kA or less [t _Al : Plate thickness of Al base material (mm)]

Pressing force (F): 9.8 x I ² x 10 ^-9 kN or less [I: Current amount (A)]

Power supply time: 600 ms or less

<Second stage condition>

Current amount (I): 2.0 to 6.0 kA

Press force (F): 0.5 to 2.5 kN

Energization time: 50 to 600ms

(I-2) When the tip diameter of the electrode chip is 6 mmφ or more and the tip R is 75 mmR or more and less than 120 mmR

<Step 1 condition>

Current amount (I): 15.0 x t _Al ^0.5 to 30.0 x t _Al ^0.5 kA [t _Al : Plate thickness of Al base material (mm)]

Pressing force (F): 2.0 x t _Al ^0.5 to 3.5 x t _Al ^0.5 kN

Energization time: 100 × t _Al msec or less

<Second stage condition>

Current amount (I): 2.0 to 20.0 kA

Press force (F): 0.5 to 3.5 kN

Energization time: 50 to 600ms

(I-3) When the tip diameter of the electrode chip is 6 mmφ or more and the tip R is 120 mmR or more

<Step 1 condition>

Current amount (I): 15.0 x t _Al ^0.5 to 30.0 x t _Al ^0.5 kA [t _Al : sheet thickness of Al base material (mm)]

Pressing force (F): 2.5 x t _Al ^0.5 to 3.5 x t _Al ^0.5 kN

Energization time: 100 × t _Al msec or less

<Second stage condition>

Current amount (I): 2.0 to 20.0 kA

Press force (F): 1.0 to 4.0 kN

Energization time: 50 to 600ms

(II) Strength level of steel: 390 MPa or more and less than 890 MPa

(II-1) When the tip diameter of the electrode chip is 6 mmφ or more and the tip R is less than 75 mmR

<Step 1 condition>

Current amount (I): 18.0 x t _Al ^0.5 kA or less [t _Al : Plate thickness of Al base material (mm)]

Pressing force (F): 1.2 x I ² x 10 ^-8 kN or less [I: Current amount (A)]

Power supply time: 400 ms or less

<Second stage condition>

Current amount (I): 2.0 to 6.0 kA

Press force (F): 1.0 to 3.0 kN

Energization time: 50 to 500ms

(II-2) The tip diameter of the electrode chip is 6 mmφ or more and the tip R is 75 mmR or more and less than 120 mmR

<Step 1 condition>

Current amount (I): 15.0 x t _Al ^0.5 to 30.0 x t _Al ^0.5 kA [t _Al : Plate thickness of Al base material (mm)]

Pressing force (F): 2.0 x t _Al ^0.5 to 4.0 x t _Al ^0.5 kN

Energization time: 100 × t _Al msec or less

<Step 2 condition>

Current amount (I): 2.0 to 20.0 kA

Press force (F): 0.5 to 3.5 kN

Energization time: 50 to 600ms

(II-3) When the tip diameter of the electrode chip is 6 mmφ or more and the tip R is 120 mmR or more

<Step 1 condition>

Current amount (I): 15.0 x t _Al ^0.5 to 30.0 x t _Al ^0.5 kA [t _Al : sheet thickness of Al base material (mm)]

Pressing force (F): 2.0 x t _Al ^0.5 to 4.0 x t _Al ^0.5 kN

Energization time: 100 × t _Al msec or less

<Second stage condition>

Current amount (I): 2.0 to 20.0 kA

Press force (F): 1.0 to 4.0 kN

Energization time: 50 to 600ms

(III) Strength level of steel: 890 MPa

(III-1) the tip diameter of the electrode chip is 6 mmφ or more and the tip R is less than 75 mmR

<Step 1 condition>

Current amount (I): 18.0 x t _Al ^0.5 kA or less [t _Al : Plate thickness of Al base material (mm)]

Pressing force (F): 1.4 x I ² x 10 ^-8 kN or less [I: Current amount (A)]

Power supply time: 150ms or less

<Second stage condition>

Current amount (I): 2.0 to 6.0 kA

Press force (F): 1.0 to 3.5 kN

Energization time: 50 to 400ms

(III-2) The tip diameter of the electrode chip is 6 mmφ or more and the tip R is 75 mmR or more and less than 120 mmR

<Step 1 condition>

Current amount (I): 15.0 x t _Al ^0.5 to 30.0 x t _Al ^0.5 kA [t ^Al : sheet thickness of Al base material (mm)]

Pressing force (F): 2.5 x t _Al ^0.5 to 4.0 x t _Al ^0.5 kN

Energization time: 100 × t _Al msec or less

<Step 2 condition>

Current amount (I): 2.0 to 20.0 kA

Press force (F): 0.5 to 3.5 kN

Energization time: 50 to 600ms

(III-3) The tip diameter of the electrode chip is 6 mmφ or more and the tip R is 120 mmR or more

<Step 1 condition>

Current amount (I): 15.0 x t _Al ^0.5 to 30.0 x t _Al ^0.5 kA [t _Al : sheet thickness of Al base material (mm)]

Pressing force (F): 2.5 x t _Al ^0.5 to 4.0 x t _Al ^0.5 kN

Energization time: 100 × t _Al msec or less

<Step 2 condition>

Current amount (I): 2.0 to 20.0 kA

Press force (F): 1.0 to 4.0 kN

Energization time: 50 to 600 ms

As described above, the present invention is characterized in that cracks in the nugget portion can be remarkably reduced by gently cooling the nugget portion after welding by making the welding current at the end of energization smaller than at the start of energization. In order to suppress it, it discovered also that it is effective to ensure the minimum thickness of the nugget part with respect to the plate | board thickness of an aluminum base material together with suppression of the cooling rate of the said nugget part more than fixed as shown by following Formula (1).

Equation 1

(Minimum thickness of the aluminum material-side nugget part / plate thickness of the aluminum base material) ≥ 0.3

Even when the nugget portion is gently cooled, if the nugget portion is too thin with respect to the plate thickness of the aluminum material, the thermal strain generated at the time of cooling cannot be sufficiently absorbed by the nugget portion. This is because cracking is likely to occur. More preferably, (minimum thickness of the aluminum material side nugget portion / plate thickness of the aluminum base material) is 0.4 or more. On the other hand, in order to make the minimum thickness of the aluminum-side nugget portion with respect to the plate thickness of the aluminum base material more than a predetermined value, for example, the current value and the energization time are satisfied so as to satisfy the following equation (2) or (3) obtained from the tendency as shown in FIG. It is valid to control.

When tip diameter of electrode chip is 6mmφ or more and tip R is less than 75mmR

When the strength level of steel is 390 MPa or more and less than 890 MPa

When the strength level of steel system is 890 MPa or more

The present invention is not particularly limited with respect to welding conditions such as other current waveforms, shapes, materials and voltage values of electrodes, and general conditions can be adopted. In addition, the welding method of the present invention can be applied regardless of the type and sheet thickness of steel and aluminum materials, and various steel sheets, plated steel sheets, etc. can be used as the iron-based materials, for example, pure iron-based materials. In addition, alloys such as 2000 series, 3000 series, 4000 series, 5000 series, 6000 series, and 7000 series can be used as international alloy symbols.

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited by the following example, Of course, It is also possible to implement by changing suitably in the range according to the meaning of the former and the latter, All are included in the technical scope of the present invention.

Example 1

The test steel containing the chemical component (mass%) shown in Table 1 was melted, and it rolled until it became a board thickness of 1.2 mm, and the thin steel plate was obtained. In continuous annealing, after annealing at 500 to 1000 ° C., quenching or rinsing is performed, and thereafter, a high strength steel sheet of 980 MPa grade is obtained by tempering. did. As for the aluminum material, two commercially available A6022 (6000 series) aluminum alloy plates having a plate thickness of 1.0 mm and 1.6 mm were used.

After processing these steel sheets (steel materials) and aluminum alloy plates (aluminum materials) in the shape of the cross tensile test piece described in JIS A 3137, spot welding was performed to create a heterojunction. In spot welding, using a direct current resistance welding testing machine, by setting the pressing force, welding current and time according to an aluminum material sheet thickness t _2, under the conditions shown in Table 2 was subjected to the welding point. The electrode chips are all domed in a Cu-Cr alloy, and include 50 mmR-12 mmφ (Table 3: Comparative Example), 150 mmR-5 mmφ (Table 4: Comparative Example), 100 mmR-12 mmφ (Table 5), and 150 mmR-12 mmφ (Table 6). , 6-shaped chips of 120 mm R-7 mm φ (Table 7) and 120 mm R-12 mm φ (Table 8) were used. The anode was made of aluminum and the cathode was made of steel, and the shapes of the electrode chips on both sides were the same in each welding example.

The nugget area cut | disconnected the sample after spot welding at the center of the junction part, embedded in resin, grind | polished, observed with the optical microscope from the cross section, and measured the diameter in the interface of the nugget formed, and calculated | required the area. The measurement measured the nugget diameter in the orthogonal two directions. In the measurement of the thickness of the interfacial reaction layer, the sample after spot welding was cut at the center of the welded portion, embedded in a resin, polished and subjected to SEM observation. When the thickness of a layer was 1 micrometer or more, it measured with 2000 times the visual field, and when it is less than 1 micrometer, it measured with the 10,000 times visual field.

In this test, as shown in FIGS. 1 and 3, the interface reaction layer is thickest in the nugget 5 center portion, and the interface reaction layer becomes thinner at the nugget 5 end portion (peripheral portion). The diameter of the reaction layer and the diameter of the interfacial reaction layer having a thickness of 0.5 µm or more were obtained and converted into areas. The measurement measured the nugget diameter in the orthogonal two directions.

As evaluation of the joint strength, the cross tensile test of the heterojunction was performed. The cross tensile test is based on the joint strength = 1.0 kN between A6022 materials, and if the joint strength is 1.5 kN or more or the fracture form is the breakage of the aluminum base material, 이면, the joint strength is 1.0 to 1.5 kN, and the bond strength is 0.5 to When it was 1.0 kN, it was set as (triangle | delta) and if it was less than 0.5 kN of joint strength.

On the other hand, the cross tensile test was used for the evaluation of the strength in the present embodiment because the difference between the test conditions is greater in the shear tensile test. The tendency of the shear tensile test was consistent with the cross tensile test results, and any of those obtained by evaluation of ○ and ◎ as the cross tensile test was high shear strength of 2.5 kN or more.

The cross tensile test results of the heterojunction after spot welding of each steel grade of Table 1 and A6022 material are shown to Tables 3-8. By comparison of the electrode chip conditions of Tables 3-8, it turns out that the bonding strength of a heterogeneous joined body will become high when a chip diameter and chip R become large in this invention range. Further, from the comparison of the spot welding conditions of Table 2 in the tables of Tables 3 to 8, by controlling the pressing force, the welding current, and the time in the condition range specified in the present invention, the nugget area and the optimum thickness range of the interfacial reaction layer It is understood that the area of (0.5 to 10 µm) increases, and as a result, the bonding strength increases.

Example 2

The test steel containing the chemical component (mass%) shown in Table 1 was melted, and it rolled until it became a board thickness of 1.2 mm, and the thin steel plate was obtained. The thin steel sheet was subjected to annealing at 500 to 1000 ° C. by continuous annealing, followed by quenching or washing with water, and then tempering to obtain a high tensile steel sheet of 980 MPa grade. As for the aluminum material, two commercially available A6022 (6000 series) aluminum alloy plates having a plate thickness of 1.0 mm (Tables 10, 11 and 12) and 1.6 mm (Table 12) were used. After these steel sheets (steel materials) and aluminum alloy plates (aluminum materials) were processed into cross-shaped tensile test piece shapes described in JIS A 3137, spot welding was performed under the conditions shown in Table 9 to create a heterojunction.

For spot welding, the correlation between the conditions such as pressing force, welding current, time, and control of the average thickness and area of the compound defined in the present invention was investigated in advance using a DC resistance welding tester. In addition, by respectively setting the pressing force, welding current and time according to the plate thickness t of the aluminum material _2, under the conditions shown in Table 9 was the welding of the first point. The electrode chips were all domed in a Cu—Cr alloy, and three types of chips of 50 mmR-12 mmφ (Table 10), 120 mmR-12 mmφ (Tables 11 and 12), and 150 mmR-12 mmφ (Table 13) were used, respectively. The anode was made of aluminum and the cathode was made of steel, and the shapes of the electrode chips on both sides were the same in each welding example. About each of these produced conjugates, the area of the depth direction thickness and the optimum thickness range of each compound was measured by the said measuring method. These results are shown in Tables 10-13.

As evaluation of the joint strength of each joined body, the cross tensile test of the heterogeneous joined body was performed. The cross tensile test is based on the joint strength = 1.0 kN between A6022 materials, and when the joint strength is 1.5 kN or more, or when the fracture shape is an aluminum base material break, ◎, the joint strength is 1.0 to 1.5 kN, and the joint strength is 0.5 to When it was 1.0 kN, (triangle | delta) and the bonding strength were less than 0.5 kN, and it was set as x.

The cross tensile test results of the heterojunction after spot welding of the steel grade of Table 1 and A6022 material are shown to Tables 10-13. In each of Tables 10 to 13, the average thickness l ₁ in the nugget depth direction at the nugget center of the layer of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound, which is the intermetallic compound layer condition of the present invention, is 0.5 to Each invention example in the range of 10 micrometers turns out that the bonding strength of a heterojunction becomes high. In addition, Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based in addition to the control of the area S ₂ of the average thickness l ₁ and Al ₅ Fe ₂ based compound layer of the compound layer, in the nugget center of the Al ₅ Fe ₂ based compound layer The invention examples in which the average thickness l _{2 in} the nugget depth direction and the area S ₁ of the layer of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound in the optimum thickness range were controlled were compared with the other invention examples. The highest bond strength is obtained. Inventive examples E, F, and G of Tables 11 to 13 correspond to this. On the other hand, in each table | surface of Tables 10-13, each comparative example which does not satisfy | fill the intermetallic compound layer conditions of this invention has low joint strength of a heterojunction. In Comparative Examples of Tables 10 to 13, the average thickness l ₁ of the layers of the Al ₃ Fe-based compound and the Al ₁₉ Fe ₄ Si ₂ Mn-based compound deviated from the upper limit and the lower limit of the range.

On the other hand, the nugget area often deviated from the recommended range described above in the comparative example. For example, 50mmR-12mmφ (Table 10) in the range of both the 12 × t ₂ ^0.5 to 19 × t ^{₂ 0.5} mm _2, also 120mmR-12mmφ (Table 11, 12), in the conditions B of 150mmR-12mmφ (Table 13) Either one is in the range of 17 x t ₂ ^0.5 to 20 x t ₂ ^0.5 mm ² , and the nugget area is small. Also, 120mmR-12mmφ (Table 11, 12), in the range of 150mmR-12mmφ the condition of C (Table 13) Any of 72 × t ₂ ^0.5 to 89 × t ^{₂ 0.5} mm _2, the nugget area is larger. In addition, in the Example, all existed in the recommended range of the above. That is, in the present invention as described above, the nugget area is preferably in the range of 20 x t ₂ ^0.5 to 70 x t ₂ ^0.5 mm ² . However, the conditions A of the comparative examples of 120 mmR-12 mmφ (Tables 11 and 12) and 150 mmR-12 mmφ (Table 13) are in the range of 28 × t ₂ ^0.5 to 38 × t ₂ ^0.5 mm ² , and conditions D, which are examples of the tables, Although the nugget diameters were almost equal to E and G, the joint strength was low. That is, as mentioned above, in order to obtain a high bond strength, it is necessary to form some nugget area, but in addition, it is also necessary to control the thickness and structure of an interface reaction layer in a joining interface.

On the other hand, the aluminum material sense yukryang, was common to each example, all minimum residual sheet thickness Δt is more than 50% of the original thickness t _2.

Example 3

The test steels containing the chemical component (mass%) shown in Table 14 were each melted and rolled until the sheet thickness of 1.2 mm was obtained to obtain a thin steel sheet. In the continuous annealing, water washing was performed after annealing at 800 to 900 ° C, and then tempered to adjust the target strength of each test steel. As the aluminum material, commercially available A1050 material (containing 0.05% Si, no Mn content) and A6022 material (containing 1.01% Si, 0.07% Mn), each having a sheet thickness of 1 mm, were used. These steel sheets (steel materials) and aluminum alloy plates (aluminum materials) were processed into cross-shaped tensile test piece shapes described in JIS A 3137, followed by spot welding to create a heterojunction.

In spot welding, the welding of one point was performed on the welding conditions shown by conditions G and H of Table 2 using the DC resistance welding test machine. A positive electrode was made of aluminum and the negative electrode was made of steel using a domed electrode made of a Cu-Cr alloy and having a diameter of 120 mmR-12 mmφ.

On the other hand, in these examples, the minimum remaining plate thickness Δt of the aluminum material was 50% or more of the original thickness t ₂ . This measurement measured the sample after spot welding in the center of a welding part, embedding in resin, grinding | polishing, and using the optical microscope.

The concentration of Mn and Si in the interfacial reaction layer was similar to the interfacial reaction layer measurement, and a buried sample was produced, and the secondary ionic strength by SIMS (ims5f made by CAMECA) was measured from the cross section. 8 keV oxygen ions are used as the primary ions, and oxygen ions are irradiated in a region of 50 × 50 μm including the junction interface to detect positive secondary ions. The intensity was analyzed line. The measurement was performed three times, and the ratios of the secondary ionic strengths of Mn and Si at the intermediate points of the interfacial reaction layer and the secondary ionic strengths of Mn and Si in aluminum and steel were determined as the concentrations of Mn and Si, respectively. Averaged.

The cross tensile test results of the heterojunction after the spot welding of the steel materials of each steel grade of Table 14 and the said aluminum materials are shown in Tables 15 and 16.

If the tensile strength of the steel is less than 400 MPa, the bonding strength is inferior, and if the tensile strength of the steel is 400 MPa or more, preferably 500 MPa or more, it is understood that the bonding strength is high. In particular, as shown in Tables 15 and 16, when the amount of C such as SPCE (mild steel sheet) is low, the strength is insufficient, and when the amount of Mn or Si is low, high joint strength cannot be obtained regardless of spot welding conditions.

In addition, when the tensile strength of the steel is satisfied according to Tables 15 and 16, or even when the steel satisfies the above preferred steel components, the nugget diameter and the area where the thickness of the interfacial reaction layer is 0.5 to 10.5 占 퐉 deviate from the claims. Low intensity On the other hand, it can be seen that the nugget diameter and the thickness of the interfacial reaction layer satisfy an area of 0.5 to 10.5 μm, and the higher the Mn and Si concentration of the interfacial reaction layer, the higher the bonding strength.

In addition, regarding the aluminum material, when Si amount, such as A1050 of Table 15, 16, is low, it turns out that the joining strength does not become high to some extent even in any spot welding conditions.

Example 4

Test steel (0.1C-2.3Mn-0.2Cr-0.32Mo) was dissolved and rolled until a sheet thickness of 1.2 mm was obtained to obtain a thin steel sheet. In the continuous annealing after rolling, after annealing at 500-1000 degreeC, it lubricated or washed with water, and it adjusted to the target intensity of 980 Mpa by tempering after that. In addition, about aluminum material, the commercially available A6022-T4 (Si: 1.01 mass%, Mn: 0.07 mass%, Mg: 0.6 mass%) aluminum alloy plate of the plate thickness of 1 mm was used.

When galvanizing steel materials, after 5 minutes of pickling and activation with 10% sulfuric acid, various platings were performed under the following conditions. Electro galvanization is performed by adding sulfuric acid to 400 g / l of zinc sulfate, 30 g / l of aluminum sulfate, 15 g / l of sodium chloride, and 30 g / l of boric acid to flow a current of 20 A / dm ² to a zinc plating bath having a pH of 3. 10 micrometers was implemented. In the Zn-10% Ni plating, 10 µm of Zn-10% Ni plating was performed by flowing a current of 10 A / dm ² in a bath in which nickel sulfate and nickel chloride were added to the zinc plating bath. Further, as a comparative example, Ni plating was performed at 10 µm by flowing a current of 10 A / dm ² using a watt bath.

Hot-dip plating was performed only on steel materials, and 10 micrometers of Al plating, Al-9% Si plating, and Zn-Fe plating (5, 8, 10, and 15% of Fe amounts respectively) were performed using various molten metals. In molten Zn-10% Fe plating, the film thickness was adjusted to 1, 3, 10, 15, 20 µm by changing the Fe component, temperature, and elevated temperature in the molten metal. In the case of galvanizing an aluminum material, after pickling with 10% nitric acid for 30 seconds, sodium hydroxide 500g / l, zinc oxide 100g / l, ferric chloride 1g / l, Rochelle salt 10g After a 30 second zinc substitution treatment in the / l treatment liquid, zinc (including zinc alloy) electroplating was performed. Each plating film thickness cut | disconnected the sample after plating, embedded in resin, grind | polished, and carried out SEM observation. At this time, three points of thickness were measured and averaged by a 2000-fold field of view. These steel sheets (steel materials) and aluminum alloy plates (aluminum materials) were processed into cross-shaped tensile test piece shapes described in JIS A 3137, followed by spot welding to create a heterojunction.

As shown in Table 17, spot welding performed three welding processes of the welding processes 1 to 3 selectively, and one welding at each welding current (kA) and each welding time (msec). At this time, the welding process 2 was set to the high current process, and the process 3 was set to the low current process. In addition, the pressing force was made into the constant value through each process. In each spot welding step, a DC dome welding tester was used and a dome-shaped electrode made of a Cu—Cr alloy and 120 mmR-12 mm phi was used in common, and the anode was made of aluminum and the cathode of steel. The nugget area of the heterojunction after welding, the area whose thickness of an interface reaction layer is 0.5-10.5 micrometers, the joint strength, and corrosion resistance were evaluated. These results are shown in Table 18, respectively.

The measurement of the nugget area and the area of the optimum interfacial reaction layer was as described above, but the interfacial reaction layer herein refers to a compound layer of Fe and Al, and Fe and Al are detected together by 1% by weight or more by EDX. The detection amount of Ni is lower than Fe and Al. That is, the layer in which Fe and Al are not detected together by 1% by weight or more, or the layer in which the amount of detection of Zn or Ni is larger than either of Fe and Al, is not referred to as an interfacial reaction layer.

As evaluation of the joint strength, the above was followed.

The corrosion resistance test of Table 18 performed about the heterogeneous joined body after the said spot welding simulated the use in automobile materials, etc., and performed the coating process after zinc phosphate treatment. That is, alkaline degreasing of each heterojunction was carried out, and after washing with water, the surface adjustment treatment was performed for 30 seconds using 0.1% aqueous solution of Surffine 5N-10 by Nippon Paint. Then, 1.0 g / l zinc ion, 1.0 g / l nickel ion, 0.8 g / l manganese ion, 15.0 g / l phosphate ion, 6.0 g / l nitrate ion, 0.12 g / l nitrite ion, toner value 2.5pt, Zinc phosphate treatment was performed for 2 minutes by the bath of 22 pts of total acidity, 0.3-0.5 pt of free acidity, and 50 degreeC. Then, it coated with the cation electrodeposition coating material (Powertop V50 gray made by Nippon Paint, Inc.), it was ignited for 25 minutes at 170 degreeC, and the 30-micrometer coating film was formed.

Corrosion resistance evaluation was performed by the composite corrosion test of these coating test pieces. The composite corrosion test was repeated 100 times by repeating the test which made 1 cycle 2 hours of salt spray, 2 hours of drying, and 2 hours of wet. And the joint part after the composite corrosion test was peeled and observed, and the maximum corrosion depth of Al was measured. Corrosion resistance evaluation was made into (circle) when Al maximum corrosion depth is less than 0.01 mm, (triangle | delta), and 0.01 or 0.1 mm or more if it is 0.01-0.1 mm.

Table 19 has shown the influence on the bonding strength by plating conditions when the various plating is performed on the steel material side or aluminum material side. Specifically, various types of plating are applied to a 980 MPa class high tensile strength steel sheet, and the conditions and welding strengths of the welded part in the case of spot welding the aluminum alloy plate of A6022 with the optimum conditions of the invention example of Table 17 are shown.

From Comparative Example 1 of Table 19, even when spot welding was performed under optimum conditions, when plating was not provided on the steel side or the aluminum side, not only the joint strength (result of cross tensile test) was poor but also the corrosion resistance was inferior. have. On the other hand, Inventive Examples 3, 4, 7 to 10, 12 to 14 are made of Zn and / or Al having a film thickness of 3 to 15 µm and a melting point in the range of 350 to 950 ° C on the surface of the steel or aluminum material. A plating film is prepared in advance. For this reason, an interfacial reaction layer can be suppressed to an optimum thickness, obtaining a comparatively large nugget area. As a result, the bonding strength is high. Moreover, since the minimum remaining plate | board thickness of aluminum material is also comparatively thick and a thinning amount is comparatively small, it turns out that generation | occurrence | production of a spatter is suppressed to the minimum amount.

In contrast, Comparative Examples 2, 5, 6, 11, 15, and 16 are provided with a coating film made of Ni, Zn and / or Al on the surface of the steel or aluminum material in advance. Out of range For this reason, for example, Comparative Examples 2, 5, 6, and 16, in which the melting point of the plating is too high, even though spot welding is performed under optimum conditions, a relatively large nugget area is obtained, but almost no interfacial reaction layer is formed. As a result, the joint strength is remarkably inferior. In addition, the minimum remaining plate thickness of the aluminum material is relatively thin, and the weight loss is relatively large.

In Comparative Example 11, in which the film thickness of the plating is too thin, similarly to Comparative Example 1 without plating, even if spot welding is performed under optimum conditions, the bonding strength is inferior and the corrosion resistance is inferior. On the contrary, in Comparative Example 15 in which the film thickness of the plating is too thick, a relatively large nugget diameter is obtained, but almost no interfacial reaction layer is formed. As a result, the joint strength is remarkably inferior. In addition, the minimum remaining plate thickness of the aluminum material is relatively thin, and the weight loss is relatively large.

From the above results, the present invention has the effect of making spot welding with high joining strength with good reproducibility and high welding strength without causing new problems such as increasing the weight of aluminum when joining different materials of steel and aluminum by direct spot welding. It can be seen. Moreover, in the transfer material body of this invention, the critical meaning of factors, such as melting | fusing point, a component, a film thickness, of a plating film previously provided before welding to the base material side, such as steel materials and aluminum materials, can be seen.

Next, Table 18 has shown the influence on the joint strength by the spot welding conditions at the time of changing the spot welding conditions to A-I of Table 17. FIG. More specifically, the conditions of the welded part in the case of spot welding and joining the above various aluminum alloy plates and A to I in Table 17 under the same plating conditions of performing 10 µm of hot dip Zn-10% Fe plating on the steel sheet. Bond strength is shown.

It can be seen that the welding strength is high when welding in which the high current condition and the low current condition are controlled in an appropriate condition range as in the conditions H and I is performed.

Example 5

The steel material and aluminum material shown in Table 20 were piled up, and continuous spot welding was performed on the conditions shown in Table 21 and the following. In addition, as an example of a temperature history, the temperature of each nugget part (central part of a cross-sectional thickness direction) of experiment No. 1 and No. 18 was measured by the method shown in FIG. That is, in the overlapped portion of the iron-based material and the aluminum-based material, in order to insert the thermocouple on the aluminum-based side, as shown in Fig. 11 (b), a groove is formed on the surface of the aluminum-based side and shown in Fig. 11 (a). As described above, the thermocouple was set at the center of the nugget and spot welding was performed to measure the temperature of the nugget. The result is shown in FIG. The cooling rate shown in Table 22 has shown the average cooling rate from 600 degreeC to 200 degreeC.

Welder: single phase rectified resistance spot welder

Electrode shape:

(Anode side) Electrode ... 1% Cr-Cu dome electrode with tip diameter 6mm and tip R 40mmR

(Cathode side) Electrode ... 1% Cr-Cu dome electrode with tip diameter 6mm and tip R 40mmR

Dome Hole: Radius 8mm

The joint seam thus obtained was cut at the weld face, and the minimum thickness of the nugget formed on the aluminum side was measured at the cross section. Moreover, the nugget part in a cut surface was observed with the optical microscope (magnification: 25 times), and the presence or absence of the crack was confirmed. In each experiment No., the presence or absence of a crack performed the welding of three samples, and evaluated the case where a crack generate | occur | produced at least one in a nugget part, and the case where a crack did not generate | occur | produce at all in a nugget part as "(circle)". These results are written together in Table 22.

Similarly, the electrode chip shape was changed as follows, and the continuous spot welding was carried out on the conditions shown in Table 24 and below by superimposing the steel materials and aluminum materials shown in Table 23. The evaluation result of the junction seam thus obtained is described in Table 25.

Welder: single phase rectified resistance spot welder

Electrode shape:

(Anode side) Electrode ... 1% Cr-Cu dome electrode with tip diameter of 7 mm and tip R of 100 mmR

(Cathode side) Electrode ... 1% Cr-Cu dome electrode with tip diameter 7mm and tip R 100mmR

Dome Hole: Radius 8mm

Similarly, the electrode chip shape was changed as follows, and the continuous spot welding was carried out on the conditions shown in Table 27 and below by superposing the steel materials and aluminum materials shown in Table 26. The evaluation result of the junction seam thus obtained is described in Table 28.

Welder: single phase rectified resistance spot welder

Electrode shape:

(Anode side) Electrode ... 1% Cr-Cu dome electrode with tip diameter of 7 mm and tip R of 150 mmR

(Cathode side) Electrode ... 1% Cr-Cu dome electrode with tip diameter of 7 mm and tip R of 150 mmR

Dome Hole: Radius 8mm

From Tables 20-22, it can consider as follows. Nos. 1 to 17 were welded by the method specified in the present invention, and no cracks occurred in the nugget portion. Nos. 18 to 28 cracked the nugget portion because they did not satisfy the prescribed conditions. That is, since Nos. 18, 22, and 26 have a one-stage pattern of energization conditions, No. 19, 21, and 24 are welded by two-stage pattern energization, but the welding current of the second stage is higher than that of the first stage. Since it is high, the effect intended by this invention is not exhibited and it is thought that a crack generate | occur | produced in the nugget part.

From the results of Nos. 20, 25 and 28, it can be seen that in order to reliably suppress cracking of the nugget portion, it is effective to secure the minimum thickness of the nugget portion as shown in Equation (1). In addition, it is understood from the results of Nos. 23 and 27 that controlling to the recommended cooling rate and securing the minimum thickness of the nugget portion as shown in the above equation (1) is effective for suppressing cracking of the nugget portion.

Similarly, in Tables 23 to 25, Nos. 29 to 35 did not cause cracks in the nugget because they were welded by the method specified in the present invention. Nos. 36 to 42 do not satisfy the conditions specified, so cracks in the nugget. This looks like In Tables 26 to 28, Nos. 43 to 49 did not crack the nugget portion because they were welded by the method specified in the present invention. Nos. 50 to 56 did not satisfy the conditions specified, so that the nugget portion cracked. .

For reference, a cross-sectional microscopic photograph of the joint seam obtained in this example is shown. Although FIG. 13 is an optical microscope observation photograph (magnification 25 times) in the bonding cross section of No. 18 (comparative example), it turns out that a crack arises in the nugget part formed in the aluminum base side from this FIG. In contrast, Fig. 14 is an optical microscope observation photograph (magnification of 25 times) in the bonding section of No. 1 (example of the present invention), but from this Fig. 14 shows that the nugget was welded well without cracking. have.

15 shows the optical microscope observation photograph (magnification 25x) in the bonding cross section of No. 25 (comparative example). In No. 25, although the cooling speed of a nugget part is slow, since the energization time in a 1st step is too long, the thickness of a nugget part becomes very thin and a crack has arisen. In contrast, Fig. 16 is an optical microscope observation photograph (magnification of 25 times) in the bonding section of No. 13 (example of the present invention). However, in No. 13, the nugget portion is secured by controlling the energization time and the like. No cracks in the

Claims (16)

A plate thickness t ₁ is 0.3 to 2.5mm of the steel material and the sheet thickness t ₂ of 0.5 to 2.5mm bonding an aluminum material by spot welding of different materials (異材) as a conjugate, The nugget area in the spot weld is 20 × t ₂ ^0.5 to 70 × t ₂ ^0.5 mm ² in relation to the plate thickness t ₂ , In this nugget, the area of the portion where the thickness of the interfacial reaction layer is 0.5 to 10.5 μm is 10 × t ₂ ^0.5 mm ² or more. The method of claim 1, The interfacial reaction layer of the bonded body has an Al ₅ Fe ₂ -based compound layer on the steel side and an Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ Mn-based compound on the aluminum side, respectively, and the Al ₃ Fe-based compound and Al Fe ₄ Si ₂ Mn-based nugget ₁₉ center nugget depth average thickness of 0.5 to 10㎛ the steel material and the aluminum material in the dissimilar material bonded body in the direction of the layer of the compound. The method of claim 2, The Al ₅ Fe ₂ system, the area of the average thickness in the range of 0.5 to 5㎛ the nugget depth direction portion of the compound layer not less than 10 × t ^{₂ 0.5} mm ₂ of steel and aluminum materials of different materials bonded body. The method of claim 2, A dissimilar material bonded body of a steel material and an aluminum material having an average thickness in the nugget depth direction at a nugget center of the Al ₅ Fe ₂ compound layer. The method of claim 2, In the interface reaction layer of the conjugate, the Al ₃ Fe-based compound and Al ₁₉ Fe ₄ Si ₂ having an average thickness in the range of 0.5 to 10 μm in a region having an average thickness of the Al ₅ Fe ₂ -based compound layer in the range of 0.5 to 5 μm. A dissimilar material bonded body of a steel material and an aluminum material in which a layer of an Mn compound is present and an area of the Al ₃ Fe compound and the Al ₁₉ Fe ₄ Si ₂ Mn compound having an average thickness range of 15 × t ₂ ^0.5 mm ² or more. The method of claim 1, The transfer material body of the steel material and aluminum material whose area of the part whose thickness of the said interface reaction layer is 0.5-10.5 micrometers is 25 * t <_2> ^{0.5mm < 2>} or more. The method of claim 1, At the intermediate point in the thickness direction of the interfacial reaction layer, the Mn element amount is 1.5 to 2.5 times the ratio of the Mn element amount of the steel, the Si element amount is 1.1 to 1.8 times the ratio of the Si element amount of the aluminum material and the Si source of the steel The dissimilar material bonded body of steel and aluminum which is 1.1 to 1.8 times in ratio with a small quantity. delete delete The method of claim 1, When the value obtained by subtracting the plate thickness thinning length from the original plate thickness t ₂ is the minimum remaining plate thickness Δt, the minimum remaining plate thickness Δt of the aluminum material at the spot weld is 50% or more of the original plate thickness t ₂ . Dissimilar material conjugate. The method of claim 1, A dissimilar material bonded body of a steel material and an aluminum material having a plating film made of Zn and / or Al having a melting point of 350 to 950 ° C. at a film thickness of 3 to 15 μm on a joining side surface of the steel or aluminum material. A spot welding method of a dissimilar material bonded body of a steel material having a plate thickness t ₁ according to any one of claims 1 to 7 and 10 and an aluminum material having a plate thickness t ₂ of 0.5 to 2.5 mm. Using an electrode chip having a diameter of 7 mmφ or more and a tip R of 75 mmR or more for both sides, a pressing force of 2 × t ₂ ^0.5 kN to 4 × t ₂ ^0.5 kN is applied in relation to the plate thickness t ₂ , and 15 × t ₂ A spot welding method of a dissimilar material assembly, wherein a current of ^0.5 to 30 x t ₂ ^0.5 kA is caused to flow at 100 x t ₂ ^0.5 msec or less. The method of claim 12, In the spot welding method, an electrode chip having a tip diameter of 7 mmφ or more and a tip R of 120 mmR or more is used for both sides, and a pressing force of 2.5 × t ₂ ^0.5 kN to 4 × t ₂ ^0.5 kN is applied in relation to the plate thickness t ₂ . And a spot welding method of a dissimilar material bonded body which causes a current of 18 x t ₂ ^0.5 to 30 x t ₂ ^0.5 kA to flow at 100 x t ₂ ^0.5 msec or less. A spot welding method of a dissimilar material assembly of a steel material having a plate thickness t ₁ of 0.3 to 2.5 mm and an aluminum material having a plate thickness t ₂ of 0.5 to 2.5 mm according to claim 11, wherein the electrode chip has a tip diameter of 7 mmφ or more and a tip R of 75 mmR or more. Using both sides, While spot welding is made up of a plurality of processes, the welding current value and / or welding time have two or more processes, and in at least one process, 2 × t ₂ ^0.5 kN to 4 × in relation to the plate thickness t _{2 of the} aluminum material. a high current process for applying a pressing force of t ₂ ^0.5 kN and flowing a current of 15 x t ₂ ^0.5 to 30 x t ₂ ^0.5 kA so that the nugget area is in the range of 20 x t ₂ ^0.5 to 70 x t ₂ ^0.5 , and As a subsequent step, a pressing force of 2 × t ₂ ^0.5 kN to 4 × t ₂ ^0.5 kN is applied, and a current of 1 × t ₂ ^0.5 to 10 × t ₂ ^0.5 kA is applied to 100 × t ₂ ^0.5 msec to 1000 × t. _2. A spot welding method of a dissimilar material assembly, comprising a low current step of flowing ^0.5 msec. A spot welding method of a dissimilar material bonded body of a steel material having a plate thickness t ₁ of 0.3 to 2.5 mm and an aluminum material having a sheet thickness t ₂ of 0.5 to 2.5 mm, wherein the welding current at the end of energization is made smaller than at the start of energization. Spot welding method. The method of claim 15, The nugget portion is formed on the aluminum material side, and the welding current at the end of the energization is controlled so that the average cooling rate from 600 ° C. to 200 ° C. of the nugget part is 500 to 2500 ° C./s. Spot welding method of the dissimilar material assembly so that the minimum thickness satisfies the following equation (1). Equation 1 (Plate thickness of the minimum thickness / aluminum base material of the nugget part) ≥ 0.3

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